Peirak's answer is too simplified and not addressing the issues. i believe you are looking more at the difference when a guy says to a girl and visa versa. aisuru (愛する) is a very serious feeling and usually reserved in japanese for great seriousness, like between husband and wife, engaged couples etc.. suki (好き) is a less serious way to say i like you or i love you. It is still a big thing to express your feelings in this way as japanese are not as open as western people in expressing their affection for someone. so to say something like daisuki (大好き) would be like saying i love you in english, and that is how it is normally translated but to express a truly deep feeling of love you should use aishiteru (愛してる) but don't go throwing this word around as easily as we throw around the word love in english.. it should be an important word for you to use.

"Suki" is translated "like"(people,things,to eat,to drink), but "aishteiru" is the feeling between man and woman - "love".