So on internal frames you don't see any metal poles at all, all the metal is sewn into the pack. Now you may be asking, well that is fine and dandy, but which is better. External frames are noted to be easier to actually pack your equipment in because they have more pockets and because it is easier to tie things on the outside of the pack. An internal frame pack is noted to be far more comfortable than an external frame. Where the external frame usually only touches your back in a few places, the internal frame is made to sort of fit straight on your back, more evenly distributing the wait.
Size is the next big issue.
The size of the pack you need depends on what you will be doing. Packs are measured in cubic inches and usually range from 3,000 to 7,000 cubic inches. Large day packs are usually around 3,000 and therefor little if you are planning on camping overnight. If you really wanted to you could take one of these packs on a longer trip, but it would be a tight fit. I took a 5,000 on a 17 day trip, and I had stuff tied all over the outside of my pack.