Your grandpa is your grandfather; in other words, your grandpa is your mom or dad's dad. Grandpa is the most common name people use for their grandfather, but you might call yours "Gramps."
While you may refer to him as your grandfather, you're more likely to call him grandpa. It's an informal, childish nickname, a shortened form of grandpappa, most popular in the 1700s, which spawned grandpappy and grandpop as well. When you are referring to your mom or dad's mother, you can replace the "p" with an "m" and use the word (yes, you guessed it!) grandma.