It basically describes the radius of curvature of the "nose" part of the bullet relative to the caliber. The higher the number, the larger the radius--the "flatter" the sides of the nose will be and (all else being equal) the longer the nose will be. It gets more complicated than that when you talk about secant ogives and offsets, but that should give you a basic idea. Here's a visual:
Many Sierra MK's, Nosler BT/AB's are around a 6 caliber tangent ogive. A pretty good compromise between aerodynamics (high BC) and not being picky about the rifles throat, seating depth, etc to provide good accuracy.
Many Hornadies (A-Max, Interbond, SST, "Spire Point"), Swift Sciroccos, old XBT's, etc use a more aggressive (~8-10 Caliber) Secant ogive. This is better aerodynamically (higher BC) but can be pickier about which rifle they shoot the best in, seating depth, etc.
VLD, ULD, get even more aggressive in search of higher and higher BC's.
That's very generalized, but hopefully it gives you a decent idea what the terms mean.