Saranghae (사랑해) is the informal version, to be used to people close to you, people the same age as you, or people younger than you.
Saranghabnida (사랑합니다) is the formal version, to be used with people you don't know very well, people who are the same age as you but of higher status, or people who work above you.
Saranghaeyo (사랑해요) is the semi-formal version, to be used with people you sort of know, and is still considered not to be good friends with.
All of the above mean 'I love you'.
Sarangeul (사랑을) means 'love'.