What is the difference between grape and cherry tomatoes?
The difference in their name is - not surprisingly - because of their form. Cherry tomatoes are round, while grape tomatoes are more oblong. Wikipedia mentions that grape tomatoes are 'sweet as cherry tomatoes', so not much difference in flavour according to this. Perhaps grape tomatoes are more similar to small plum tomatoes.

This site claims grape tomatoes to be sweeter, having a thicker skin and a lower water content than cherry tomatoes. Interestingly, it also says that grape tomatoes are a cross between cherry tomatoes and other tomatoes.

The claims of the second site are in agreement with this site ("grape tomatoes have thicker skin, lower water content and intense sweetness"). It also states "a grape tomato is half the size of a cherry tomato", so perhaps they are smaller. Due to the lower water content, grape tomatoes have less chance of 'squirting' when being bitten into.