Pizza sauce and spaghetti sauce are both tomato-based, have roughly the same seasoning (Italian) and look a like. However, I (and a friendly user here) think there is a difference between the two. So what is it? Or isn't there one and you can interchange the two without a weird feeling or taste?
Red pizza sauce is often (but not always) two things:
Thicker. Thinner sauce will tend to run in the oven and also steam the pizza crust as it cooks - if loaded with toppings, otherwise thin is fine. Depending on the crust, the heat of the oven, the toppings above sauce, and how watery it is, this may not be needed. If you've just got some crushed tomatoes and a few bits of cheese in a super hot oven - the sauce should be fine without reducing beforehand. If you've loaded up a bucket of sauce and a pound of cheese, precook and reduce the sauce.
It's often simpler. Many pasta dishes like spaghetti highlight the sauce and hours of simmering happen for the sauce to bring it to perfection. They're all about the sauce. Pizza is really about the crust and what highlights it. Many of the best pizzas are simply topped. Crushed san marzano tomatoes (sometimes) reduced with a hint of salt is often all I do, and franky, it's enough. I'm not talking about the jarred varieties here, but what you might cook at home. Jars in the store, the main difference is just how thick it is.
When you ask about "pizza sauce" I'll have to assume you mean the tomato-based sauce that became the norm for just about all pizzas in North America since about 1955 when we crawled out of our meat & potato caves and started to try new things. (Thank you E.D. & J.C.)
The reality is that there really isn't a "pizza" sauce. There is pizza and whatever the toppings are of your choice. Example: Basil pesto, bechemal, tapenade, olive oil, heavy cream and tomato sauce used for bases to name only a few.
However, for the tomato-based pizza lovers out there here's a FYI...
Restaurants don't have a pizza sauce and a pasta sauce...they have a single tomato sauce used like you would use a stock - as a base to build from for other items.
What is the norm for most restaurants is a traditional house-made tomato sauce. Tinned plum tomatoes(tinned are actually better than fresh), onion, garlic, oregano, basil, sugar and whatever secret item the chef uses, cooked for anywhere from 30 minutes to 4 hours until the desired consistency is found. Typically it will be blended to make it smooth and easier to use in other dishes or as a possible base for pizza if required.
The pizza dude down the street is probably using canned Ragu or Hunt's tomato sauce for everything he makes.
The difference in the flavours: texture will be a function of what has been added to the dish and the cooking time/method. You'll see reduction in the sauce of a pan-made-pasta-sauce and evaporation/caramelization happen with the sauce on a pizza. Both yield a difference in flavour and texture that will make you think they are two different sauces...which they are once you've added things to them, but both have started from the same batch of cooked tinned tomatoes.
And so the veil has been lifted...
Tomato suace in a pizza parlour is typically prepared uncooked. Since pizza ovens can reach temps in excess of 800 °F (430 °C), the sauce cooks on the pizza. Precooking sauce can "overcook" in the pizza oven.
Also, traditional Neapolitan pizza philosophy is "less is more", so the sauce tends to be very basic with a few to no ingredients added. I add oregano, salt and pepper to a can of crushed tomatoes (although lately I omit the salt for my hypertension). I've even steered away from oregano as it can turn the sauce bitter if not used within a day or two. I'll substitute with parsely. When I make a Pizza Margherita I don't add anything to the sauce. I simply sprinkle fresh basil over top of the pizza with fresh mozzarella. Sometimes I don't use sauce at all: just thinly sliced tomatos. That's the beauty of pizza: no rules!!!
Here's my Paulanardi Pizza Sauce Recipe:
- Can of crushed Tomatoes.
- Tblsp Parsley.
Pinch each of sage, rosemary, thyme
Get it... Paulanardi... Paul and Artie... Simon and Garfunkle... Parsley Sage Rosemary and Thyme.... GONG....
- I might also add some garlic powder and sugar.
Both are just a reduction of fresh tomatoes over a a period of time (30 mins to 4 hours or more). Simple flavours may or may not added during cooking, or at the end of cooking
Pizza sauce is intended for smearing onto the uncooked pizza base, and must have a consistency suitable for doing this
Pasta sauce is intended for being soaked into some nearly cooked pasta so it infuses and coats the pasta, usually just before serving
From the perspective of a person that understands Italian cooking or just cooking in general, sauce used on pizza and pasta is diverse. More or less any sauce you use on one could be used on the other with a few modifications depending on the other attributes of the dish.
From the perspective of American consumer marketing the answer is a lot more black and white: The red stuff in the jar labeled as "spaghetti sauce" is typically thinner and sweeter with more basil, perhaps with pieces of other vegetables, mushrooms, or ground meat floating in it. The stuff in the jar labeled "pizza sauce" is generally thicker and less sweet with more oregano and doesn't have chunks of other things in it. Both are tomato based, but with different flavors and consistencies to match the average Joe's perception of what the sauce should be like.
For some, tomato-based (red) pizza and pasta sauces are interchangeable. Others note differences, but aren’t sure what they are. This tells us right away that the two sauces are indeed somewhat similar.
For those who make these sauces often, there are certainly differences. Of course, every chef had their own recipe and, so there is a lot of variety. For this reason, I’ll list the general differences betwen the sauces, rather than listing distinguishing characteristics or hard-and-fast rules.
- Red pizza sauce is almost always thicker than pasta sauce, for reasons mentioned by @rfusca in their answer.
- Pizza sauce tends to have a zestier or spicier flavor than many—but certainly not all—pasta sauces. In particular, it tends to have more fennel, red chilies and/or cayenne, and certainly more oregano than generic pasta sauces. (Some say pizza sauces have more garlic, too, though this is different than my own experience and style.)
- Pizza sauce usually has a significant (noticeable) amount of grated parmesan cheese in it. Pasta sauces may have various cheeses, but they tend to be melted into the sauce (for lack of a better word)—this is in part because of the difference in preparation styles: longer cooking times allow pasta sauces to melt the cheese.
- Pizza sauce is often very slightly sweeter than authentic pasta sauces. Pasta sauces get their sweetness from vegetables and/or carmelization processes (see next bullet point), but pizza sauces pften have added sweeteners like ground sugar or honey. (Note that canned or “tinned” pasta sauce manufacturers often add sugar to their sauces to mask the acidity for the same reason.)
- Pasta sauces often focus on subtle flavors developed through long or involved cooking processes. For example, carmelized onions, carrots, bell pepper, and other vegetables may be used in or as flavorings for pasta sauce, but never (?) for a pizza sauce. Pizza sauce is often put on pizza without cooking it first.
The closest pasta sauce to a classic pizza sauce is a marinara sauce. In fact, some pizza parlors offer marinara dipping sauces for their crusts or allow patrons to have marinara sauce on their pizza.
I use sweet Philippine style spaghetti sauce with pizza spice added to it. Some garlic powder added. It is a little damper than regular pizza sauce like in America - but our cheese is drier than there and the thinner sauce helps melt the local cheese. Also some times add a touch of ginger. Pizza sauce is hard to find were I live. Or a pizza shop.