some forms of eyeliner are liquids. you can either get "liquid eyeliner" which is a liquid or "eyeliner pencil" which is a solid.

Liquid Eyeliner on her top eye lid.

Well liquid eyeliner is really whatever is in eyeliner but not formed into a solid just poured into a bottle..which i guess is really Kohl (a powdery substance)

Liquid eyeliner is the hardest form of eyeliner to work with. You will need steady hands to achieve it on the first attempt. If doubtful, draw a thin line of your desired shape for eyeliner with your usual pencil as your base first then go over it with the liquid liner.

In some ways, yes. Most people find liquid eyeliner easier to apply and it gives a better look, especially if you are going for something dramatic. Liquid eyeliner, however, comes in small bottles and therefore runs out more frequently. So if you are buying a very expensive brand of eyeliner, it could be very expensive to keep buying liner. Also, liquid eyeliner smudges a lot more quickly and easily than pencil eyeliner does.