So I am so confused on the card CELL PORTAL, can someone please tell me how this card is used and when it would be used. Thank you.

It is used to make the runner go back to the start of the run. So if for example you have a well developed server with 2 or 3 other ice with cell portal as the last one you could force the runner to go all the back to the start and have to break those ice all over again. Most likely this would result in the runner jacking out as they are out of credits possibly or cannot afford to take some damage from subroutines they will re-encounter or stuff like that.

Nice with automatic 'damage' dealers like DATA RAVEN in first row…

 Sending the runner back to a Tollbooth or Data Raven is always fun. Too bad the runner can just jack out before encountering the first ice after getting Cell Portaled though. 

How does this play out if its your only piece of ice? I imagine it gets flipped over after the subroutine goes off and your forced to re-rez it, making it a horrible front line defender.

Alternatively, what happens when you have a wall of thorns as your outermost ice unrezzed?  I.e. I can't afford to rez wall of thorns so the runner avoids it approaching the cell portal. I rez cell portal and its subroutine activates derezing it.  Now I have no rezed ice and no credits.  Does the runner then pass over both pieces and access the server?

Cigar said:

These questions only seem to make sense to me if you wrongly assume that you must rez unrezzed ice when the runner approaches it, if able. You can always decide whether to rez an unrezzed piece of ice that the runner approaches.

So this ice isn't supposed to be the first ice a runner faces in a run, right? Sorry for the newbie question, my game arrived today.

Cimmeria said:

Right. This piece of ice obviously makes the most sense being the innermost ice, so that when you send the runner back to the outermost ice, they have to deal with all the other ice on the server again.