Back in the old days, when Robbins was a climber, the hardest grade given to a climb was 5.10. It was a closed scale. So, 5.11 was kinda like Nigel's amp...better than everything else.
Now, 5.11 is pretty mellow by most standards. As climber's started to really get better, the scale needed to be opened and currently it tops out at about 5.15a or b. The letters refer to finer gradients in difficulty that are used on the 'higher' end routes (starting at 5.10) and go a, b, c, d and then the next number. The whole thing is pretty subjective and frustrating to alot of people; it's hard to attach a single number to a climb that covers alot of different types of moves and angles.
Currently, the 'hardest' routes are graded 5.15. There are a few touted to be this grade, but 'Realization' established by Chris Sharma is currently laying some heavy smackdown on the rest of the world. This route took Chris a coupla seasons to complete and had been bitchslapping top climbers for about 8yrs before it was done. Also, Fred Rhouling's (FRA) route 'Akira' hasn't been done again despite much initial criticism and doubt of it's difficulty.
Chris Sharma on 'Realization'
This route is over 150' long and overhangs by about 30-40' in that height.