Salsa in Mexico is normally used for calling a spicy hot dressing that you will put in the table so people can serve them selves and spread it over their dishes or inside their tacos. As we do it in the center of Mexico it must be very (spicy) hot, there are different kinds: fresh, cooked: boiled, grilled.
The most common used fresh salsa is the Pico de gallo also known as Salsa Bandera or Salsa Mexicana, these last two names refer to the colors of Mexico's flag.
Pico de Gallo ingredients are: Chopped jitomate (oval tomato with belly button), chopped onion, chopped serrano pepper (or green Jalapeño), choped cilantro leaves (fresh), juice of one sour lemon (lime), a pinch of salt. You mixed them and serve in a small bowl.
A boiled / grilled red sauce can have the following ingredients:
4 Serrano peppers (you can vary the number depending how hot you want it)
2 Jitomates (red oval tomato with belly button)
1 quarter of onion
1 garlic clove
Fresh Cilantro just a bit (2 table spoons)
1/2 cup of water
You need to bring the peppers and tomatoes to a boil until they are very tender or toast them in the grill/pan until the skin starts turning to black, if grilled you may want to remove the hard skin from the tomatoes after grilled. If you boiled them you can reuse the water in the next step.
Then you put everything in the blender, if you want it to be very liquid you use a high speed if you want it to be thick use the lower speeds.