PLEX in EvE makes the virtual world go round. You can use it if you're strapped for cash, by buying some with your Dollars, Euros, Yen, …Whatever it is you use, and then sell it on the market for ISK.
The ISK worth of it right now is 710million, give or take a few.
If you have the ISK for it, you can use it to pay for your subscription, one PLEX being the equivalent of 30 days of game time.
You can even buy them and just put them someplace until they're worth more than they are currently. Which is something that a lot of people have been doing since they just keep going up. I myself am guilty of this too.
PLEX has other uses, there's the Aura store that I know absolutely nothing about. There's the dual training that's rather new.
And since a few days you can also buy the collector's edition with PLEX...
I'm sure I left out a few things but there's not many other games that allow you to use in game currency to buy things like this.
Which makes me come to my little milestone. For the first time in my EvE carrier I have used a PLEX to prolong my game time.
I'm pretty proud of myself. Not only have I used my ISK, I have stepped away from me not wanting to use said ISK on something for myself.
"PLEX going to a good cause... Me"