The short, informal way to express the difference is that, in BCNF, every "arrow" for every functional dependency is an "arrow" out of a candidate key. For a relation that's in 3NF, but isn't in BCNF, there will be at least one "arrow" out of something besides a candidate key.
Wikipedia entry for 3NF table not meeting BCNF
A common misconception is that you can normalize to 2NF and no higher, then to 3NF and no higher, then to BCNF and no higher. In fact, fixing a partial key dependency in order to reach 2NF often leaves you with all relations in 5NF. That is, you went from one relation in 2NF to multiple relations in 5NF without stopping at BCNF in between.