"He moves like a snail" Using the word like, you are able to compare the boys speed to the speed of a snail. Using a simile you can see just how slow the boy moves.
"She is as tall as a giraffe" Using the word as, you are able to compare the girls height to that of a giraffe. The girl and the giraffe are not similar at all but by using a simile you can see how tall she is.