My grandma told me its a good idea to save the bacon drippings in a sealable container to cook with later. I remember when I used to watch her cook with it, it was always solid. I have started saving the fat from my bacon, only the bottom of the can is the only part that ever congeals. The top always seems sort of semi-liquid. Is that ok? When cooking with it, what part should I use and what is the difference between solid and merely viscous bacon fat?
To answer what I think is the question (you put all of the grease into a container and there's a residue at the top), bacon drippings are not 100% fat. There are also solid pieces of bacon in there and other "impurities" from the curing process.
When rendering bacon fat, you should line the container with a paper towel first (or cheesecloth if you have it). Pour the bacon drippings onto the paper towel and the fat will drain out the bottom; the solids will be left behind and you can dispose of them. You'll be left with (mostly) pure fat.
The rendered fat will most definitely congeal; the vessel, once cooled, should contain only a solid, off-white substance.
In your grandmother's day, companies didn't adulterate bacon by injecting it with Lactose (Which causes the bacon to absorb water, so you end up buying the meat with sometimes up to 25% of the weight being water)
This is why when you fry bacon you often get a white slushy residue leaching from the rashers, and you end up with poached bacon, rather than fried.
Unless you buy your bacon from an independent producer (and pay the price premium) I doubt if you will ever be successful in rendering the run-off.
Pure bacon fat is always solid at room temperature. For a while, this was part of the nutritional justification for why all saturated animal fats were unhealthy, because they would be 'solid' inside your arteries as well. That picture is much more complicated today, but suffice to say that the physical trait of solidness at room temperature hasn't changed.