Ratings are subjective and minimally useful, but if you must know, there are two nearly identical rating systems for bouldering: the V System, and the B System. The more standardly accepted V System was invented by John"the Verm" (V) Sherman and goes from V0 on up. Today's hardest problems are pushing the V14 range. A V0 is the rough equivalent of 5.10 climbing, while V5 boulderers can expect moves that feel like 5.12. The hardest boulder problems would be off the charts on the traditional Yosemite rating system if they were long enough to comprise a pitch of climbing.
The B System, also known as the Peak System, goes from B0 to B14 and also starts at around 5.10 difficulty. There is little difference between the two, except that the V System allows for slightly more specificity in the lower-end ratings and in the mid-range difficulties.
Then there is the French system, originating in the bouldering Mecca of Fontainbleu. You won't see this one used inside the U.S., but in case you do, it runs from "5" on up to about "9", with letter subdivisions breaking up the grade. For instance, a V6 problem would be about 7a+ on the Fontainbleu scale.
Details mentioned in this article were accurate at the time of publication