I understand the OS is modeled over UNIX/Linux - if this is the case, when I open a Shell, would I also use the same commands that I used to use when I had Linux?
MacOS X (10.5 and 10.6) are certified UNIX's (linux is specifically not, and will forever be "unix-like").
As to "the same commands as Linux", then this is a definite maybe. Outside of the POSIX/UNIX mandated commands, MacOS X more often falls back on the BSD forms of doing things, rather than the GNU (linux) versions. While the broad strokes, and most of the commands are similar, the details can be annoyingly different if you switch back and forth all the time (different switches, different options, etc...).
Don't fall into the trap of thinking that one is better than the other. They are just different.
The other thought trap to avoid falling into is that MacOS X is not "built on top of" UNIX. Rather there is a BSD API layer in there. At the bottom of things the BSD layer is built on top of a (heavily modified) Mach kernel, and all of the very bottom layers (drivers, etc) are very different than the way linux puts them together (or traditional BSD for that matter). Additionally the top layers (the GUI) have only a few places where they really tie into the BSD layer (filesystem being the big one), mostly they are connected more directly with Mach.
And on the subject of shells, most of the usual ones are available: sh*, bash (default), tcsh, zsh, etc... More esoteric ones are available.
* as most *nix's today sh is just the bash binary running in a special mode
