I would like to add some nuance to what people already said. For "aimer" you can also use it with family or pets. If you say to your mother/brother/children "Je t'aime/vous aime", it means that you truly care about them. You can use it for friends by adding "bien" : "Je t'aime bien !" means you appreciate someone as a friend. When someone say "j't'aime" or "j't'adore" it's familiar. In SMS language, you can also see it written that way : JTM (je t'aime) JTD (je t'adore). To "adorer" is something stronger than to "aimer bien" for a friend, if they say "je t'adore !" to a friend, it's because this friend/person has just made something amazing, like solving a problem of them. We can also say " tu sais que je t'adore, toi ?"

J'aime (aimer, vouloir) : I like to do it that way. For example: I don't like him. Je t'adore : I love you (aimer beaucoup, avec passion), with a friend for example.

Je t'aime = I love you in a romantical way Je t'adore = I really like you, but you only say that to your friends since it doesn't show any romantical attraction But if you say : 'J'aime le rugby' et 'J'adore le rugby', if you use the verb 'adorer', it is stronger than 'aimer', so pay attention.