What exactly are gel nails?

Okay, so I just got gel nails done, but the lady who did them wasn't very fluent in English and had a hard time explaining to me what they were, so I didn't keep asking. I went in with the impression that a "fake" nail would get put on my natural nail to add length (like acrylics) so I trimmed my nails quite a bit before going in and when the technician saw my nails she did say something along the lines of " They were very short, but she could make it work". I was kinda confused but just went with it. So long story short, I ended up with my Short nails in a french style but with the gel on them. It basically looked like a manicure, and I really wanted long nails. So fast forward a week, I see my boyfriends sister with the most natural looking gorgeous nails and asked if they were acrylics and she tells me "No, they're gel" but that they added nail on hers b/c hers were fairly short. Then I hear that you have to go in with your natural nail the length you want and they will shape and add gel to that. I am SO confused! Someone please explain it to me. Thank you guys in advance and sorry for the novel sized preface. :)