mephedrone is 4-methylmethcathinone (4-MMC), or 4-MMCat, which obviously gave rise to the "meow meow" appelation. It's also called "bubbles."
The drug is a stimulant of the amphetamine class and has not been banned (yet) because it is still under review and virtually untested. It is illegal to sell it for human use, so it is sold over the internet as "plant food."
An article this past week in the Times lists some of the side effects and complications, including one apparent death, and a case of young man who in a state of disorientation ripped off his own scrotum.
For those interested, there is additional information in articles at Gawker, Faster Times, and Vice.