Nachos are a Tortilla chips that is topped or served with melted Cheese and chilies, most often slices of Jalapeno.  Nachos can also be topped/served  with freshly made chopped tomatoes (Salsa) or tomato-based sauce as in the picture below. 

Nachos is a food from Northern Mexico where Tortilla chips are  called Totopos (crunchy corn Tortilla chips). The dish is created by Ignacio Anaya, a Mexican.