I am running a long run (approx. 250-275' to a switch and then another 250-275' to another switch) Each end switch will be connected to a computer. I would like to know which type of cables I should use. The wire will be sealed inside PVC pipe and buried. The total run will be approx. 500' with a switch in the middle because I was told that I needed one. I am confused about the solid, stranded, Plenum Cat5, Cat5e, Cat6. Any help would be greatly appreciated. Thanks.

if you're doing work like this, chances are you probably need certification (or should)
check a CCNA book or something

That's quite a long run. UTP cable runs should be limited to 100m and looks like you are close to that limit.

Do you know why you need the switch in the middle? Will be be connecting to other equipment or is it just to join up the two segments?

If that is the case, an alternative (although probably more expensive) is to use fiber optics cable and RJ-45 to Fibre Optics media converters.

Or a simple repeater. No need for fiber for a single pc...unless there is room for growth in the future.

Only problem with this is that any interference will be broadcasted with a repeater, which can mean lost packets. With a switch you would be safer. Also, the type of switch you decide to purchase will be important. The quoted length is 100m but I've seen problems with Linksys and D-Link to name a few, that have problems after 100ft. You could also go Directional Wireless if it is a clear line of sight between the 2 locations. As far as cable goes, different shielding is usually for code regulations, and other names (Cat5, Cat5e) are for transfer speeds, but most cat5 should work.