Well, a "grape" is just one grape. "Grapes" is the plural form, and refers to more than one grape. We usually talk about "grapes" (since grapes are small and come in bunches, so we usually eat more than one), but you should say "grape" if you are only talking about one grape. You can also use "grape" as an adjective to describe things that are made from grapes, or flavored to taste like grapes. "Grape juice" is juice made from grapes. Examples: "Grapes are a type of fruit." "You can add more fruit to your diet by eating more grapes." "Should we buy some grapes from the store?" "That grape fell on the floor. Throw it away." "Let's give each of the children one more grape." "There is a bottle of grape juice in the refrigerator."

grapes is the plural form of grape. grapes is used when you have more than 1 grape. ex: after lunch, i had 5 grapes. ex: today i only had one grape, it was delicious.