DrIstvaan: Since there isn't a place where you can get a list of symbols and what they actually do (or at least none that I know of), I decided to start a thread to collect them. Please, if you have more or can give info on the ones I don't know, add your contribution to this list.
Here are the ones I've found so far.

Heart - Gives you two hearts
Satanic - Poisons enemies around you
Time - I guess it extends the time available to finish the level, but I couldn't prove it yet
Ghost - ?
Eye - Temporarily freezes enemies
Pentagram - ?

tinyE: I can help with one, Ghost. That means hurry up! :D As far as I know you can kill them with the Bubble Gun. If you don't have one of those get your ass to an exit!

DrIstvaan: Why would a symbol hurry up the arrival of the ghosts?
I know there is a red hourglass which does that, but I doubt there is such a negative-effect symbol...