What's the difference between "I'm in love with her" and "I love her"?

The use "in love" indicates a romantic affection, while "love" can also be platonic, between family members, towards pets, or even towards inanimate objects.

If the object for the affection in your sentences was a boat, the second sentence would make perfect sense, but hardly the first. (The first sentence could be used that way, but then it would not be literal, but rather describe a feeling so strong that it would be comparable to romantic love, but still not being of the romantic kind.)

To add a psychological or spiritual perspective to this question, the two phrases can be used to contrast true love and false love.

When you are in love with someone, the object of your adoration serves the mere purpose of satisfying your own emotional needs. When you truly love someone, you care about the person's needs as a separate individual distinct from you.