Pico de gallo is a fresh, uncooked condiment made from chopped tomato, white onion, lime juice, fresh cilantro and chiles (typically jalapeños or serranos). Other ingredients may also be added, such as or apple cider vinegar, cucumber, radish or firm fruit such as mango.  Pico de gallo can be used in much the same way as other salsas, but since it contains less liquid, it can also be used as a stand-alone side dish and as a extra tasty  ingredient in dishes such as tacos and fajitas. 

It is sometimes called salsa bandera (flag) or salsa mexicana, because the colors – red (tomato), white (onion) and green (chile and cilantro) – are the colors of the Mexican flag.  I have actually never heard anyone call it pico de gallo; it's always called bandera by my friends.