Jeffersonian has a fantastic reloading tutorial on his website. It is very comprehensive and has many photos to help explain how it all works.

It is a great read. I highly recommend it.

I’ve received a lot of charity, some of it shockingly spontaneous, from the Gun Culture. I therefore felt motivated to give something back, so I created these pages, showing how (and why) I reload ammunition.

The main reason to load your own, especially for rifle cartridges, is to save money; when I first started shooting competitively in 2003, I was using either Berdan- (and corrosive-) primed surplus ammunition in my Mosin M44 carbine, or expensive factory ammunition in my VZ24 Mauser. In a match in February 2004, I won a gift certificate for a set of Hornady reloading dies at a Vancouver-area gun shop; I chose 7.92x57mm (“8mm Mauser”) for what was at that time my best rifle, the VZ24. At the time I calculated that, ignoring the capital cost of equipment, tools, and used cases, and counting only the cost of consumables (bullets, primers, powder), I was paying about 30¢ per reloaded cartridge, vs. 75¢ for factory rounds. So there’s your motivation.

Other reasons to load your own are to control the precision of the end product, for better accuracy than the factories produce; and, to make a load that the factories don’t offer, like the superlight 12 gauge shotshells I make for my antique side-by-side shotgun, to vastly reduce both chamber pressure and recoil (which are not necessarily connected to each other).

More here.