I'll put this out there first and foremost. I am not a PLEX trader. I have bought some in the past, but generally, I steer clear and make my ISK with manufacturing. That being said, PLEX is big business. If you don't know, PLEX is a pilot license extension. You turn it into CCP on your account and are given 30days of play time in lieu of paying the 14.99 fee for your subscription. This allows people to play for free if they use ISK to purchase game time. A great idea, right? Well, yes and no. Let's review.
PLEX is a great way for people who don't have a large portion of their income to pay for subscriptions. As CCP allows for multiple accounts, it can be a great way to defray the costs of multiple accounts or use them to completely pay for your subscriptions. PLEX are a commodity that you are able to buy and sell on the market place. They are not seeded by CCP normally so all the PLEX on the market have been purchased as a RMT. There has been some that were gifted, but 99% of them are player purchased.
Being a commodity, they are subject to supply and demand. Their price ebs and flows with the supply. Some markets are less expensive than others and places like Jita and Dodixie have a high sales volume. This is all pretty damn cool. I don't know of another MMO that allows you to trade your cash using PLEX as a proxy. The problem is it is expensive, a good target for gankers and it caused an influx of available ISK. This by itself isn't horrible, what it proliferated was new players in bling ships who get ganked.
I've seen it all to often. New players quickly train up to battleships, buy faction ships, fit them with faction modules and wander haplessly into low sec or get ganked in space and lose hundreds of dollars of real life cash. The problem is that PLEX promotes the misguided use of things that veterans might not do. I've seen killmails that would make any pirate drool over. Deadspace/Officer/Faction fit Vindys with 6bil killmail losses, capitals in the 10's of billions and on and on. Being new players, they may not realize yet how to properly fit a ship or know where not to fly it.
I'm all for PLEX, don't get me wrong, but if you are a new player, I would highly encourage you to learn the basics first, join a corp of helpful people and ask questions before you drop some hard-earned cash on PLEX.