SPST: single pole, single throw. Poles refer to the contacts, i.e. one set. It will have three contacts- the middle is common and the circuit is completed with either of the outer contacts depending on switch position.
DPST: dual pole, single throw. Six contacts, but it will basically have two separate cirtcuits like above, so the the two sets of wires going through are isolated- just like having two separate switches at once.
I wouldn't recommend buying switches (or any components for that matter) at Radio Shack- they're invariably junk, especially for a switch that will see a lot of use and outdoors, at that.
Best source would be Digi-key or Mouser online. NKK, Mountain are all good; in fact they don't sell anything that isn't good. Mouser will send you a huge catalog free. Both Mouser and Digi-key have great websites that let you see PDF's of the catalog pages, but it can be a little tricky to browse if you don't know exactly what you want.
Finally, pay close attention to the physical size and amperage rating as you'll want a pretty beefy switch for this application due to the current and wire sizes (i.e. you don't a miniature or sub-miniature toggle).