My knowledge of such things is quite limited compared to many folks here, but I'll help where I can.

On calibers:

The caliber of BULLET used is in decimal form. For instance a 300 caliber (be it a 30/06, 300 Win mag, 300 Wby Mag, 308, 30-30) is (in theory) 30/100 of an inch in diameter. As it turns out, since the same sized bullet are fired from many different CARTRIDGES, we can't call them all the same thing. In reality, each of the listed cartridges fire a bullet that is a .308, or 308/1000 of an inch in diameter.

The main differences in each is simply the shape of the brass (spent casing) and how much powder it can hold.

The same principles apply to millimeter designated bullets. A 7mm would equate to a .284 caliber, and so on and so forth.