Cassettes and Freehubs
(c)2003 m.s.gerritsen

New derailleur equipped bicycles will nearly all have a freehub nowadays. With older bikes the situation is more confused. Below we give a few pointers how to spot what you have.
This is a cassettehub taken apart. The freehub (black thing in the middle) is placed over the aluminium serrations on the hubbody, and tightened with the central bolt. The stack of cogs is slipped on the freehub, and tightened with the lockring

This is the classical solution with threaded hubshell and freewheel. Here too you can change the cogs, but you will need the right ones. On this particular freewheel 5 cogs are fitted on splines (two different diameters) and the three threaded cogs are all different!. To reach the spokes on this hub you have to remove the freewheel with a freewheelremover and a very large wrench.
This is what a standard Shimano Hyperglide cassettehub looks like. The -black- lockring keeps the stack of loose cogs on the hub. If you unscrew the lockring ( you will need a tool which engages with the serrations) the cogs will slide off.
A predecessor of the hyperglide freehub was the uniglide hub. Here the cogs are kept in place with the threaded last cog. Uniglide cogs have an oldfashioned toothform with high teeth, and the cogs are reversible. Compared with modern offerings they were everlasting, are still much cherished by tourist, and of course discontinued (sigh). 
To remove the cogs you will need two chainwhips or on the road the Pamir cassettecracker.
Cassettehubs often have a bulge in the hubbody to provide strenght around the hollow bolt keeping it all together
A freewheel on a corresponding hub. A giveaway is the deep hole from where the axle protrudes, and the small cog which is screwed in the next one. Another giveaway is a name from the past as Maillard, Regina or Atom.
This is how a lockring is removed. This lockring remover has it's own pilot engaging with the axle, cheaper versions are kept in place with the QR skewer done up loose.
The chainwhip prevents the freewheel from freewheeling, and with a large wrench you unscrew the lockring (ACW= normal thread).