Tightline is similar to the lower waterline, except the eyeliner goes above your eye - on the upper eyelash waterline, as pictured here:
It's taken me a while to master eyeliner, starting a little at a time, and over the years gradually having the confidence to try full winged eyeliner etc. There are many tips and tricks to do this, but this was one of the best beauty related tip I've heard of, and you can use any gel, cream or pencil eyeliner you desire, (liquid eyeliner is generally too wet for this technique). It's like fake eyelashes, without the fake eyelashes.
As you can see, this technique makes the eyelashes look really dense and full in the after shot.
Do you use tightlining?