The previous answers are dead-on. The forums are best suited for answering a specific question. Also, there is a good chance that anyone's attempt to answer general questions will be incomplete, and possibly dangerous.
Nevertheless, I'll give you a summary
, with the understanding that a more complete treatment is available in reloading manuals and texts -
- Size - If you mean "degree of crimp", this is like Goldilocks (not too hot, not too cold, just right.) Excessive crimp can bulge the case and cause chambering problems. Insufficient crimp can cause things like bullet pull (in revolvers) or setback (in rifles with tubular magazines), and also cause chambering problems in semi-auto pistols if the bell isn't removed.
- Dies - There are combination seat/crimp dies and separate seat and crimp dies. It is generally easier to use separate dies, as the two actions "fight"' each other to some extent in a combination die.
- Crimp - Forming the mouth of the case using a die.
- Roll Crimp - A type of crimp which "rolls" the mouth of the case into a groove or cannelure in the bullet designed to receive it. Roll crimps applied to bullets without either of these usually damage the bullet and degrade accuracy. Usually used with revolver cartridges, especially magnums.
- Taper Crimp - A type of crimp which removes the "bell" created by an expander die to make bullet seating easier. Taper crimps should not roll the case mouth. Usually used with semi-auto pistol cartridges.
And, for completeness, there are two flavors of Lee Factory Crimp Die which need to be separately researched and judged on their own merit.