First of all, they're padstacks, this is the basic building block.
You can use a padstack to become a mechanical pin, an electrical pin, a via, or a blind and buried padstack.
Allegro is set up to be used in a structure where people share the same libraries, of course you want the padstacks defined externally and in a library. It becomes VERY MESSY VERY QUICK if you have all kinds of padstacks with wildly different definitions and drill sizes.
An Allegro .brd file is a database that contains not only the layout itself, but all the constraints, symbols, and padstacks for that design.
You can export all this either from Allegro itself or from the command line (system command line, not just Allegro's)
So if you like a footprint you found on someone else's PCB, you can dump it, along with all the padstacks.
Allegro originally didn't have an undo/redo feature. The way it works is you start a command, you are in that mode, as long as you're in that mode you can oops/redo one step at a time, but when you click DONE, you were DONE. No going back!
Allegro now has undo/redo, but it undos/redos the entire command sequence. Like if you route 35 cline segments between two pins, hit DONE, then you want to just undo the last segment and hit UNDO, it will undo the entire 35 cline segments.
But don't worry, once you understand this, it doesn't matter!
If you want to delete just a segment in your cline (the cline being the connect line between two pins, including all the bends), just select edit->delete, or the big X in the toolbar.
Then select all off and Cline Segs in the find filter. If you can't find the "find filter", view->windows->find.
Now you can click on individual segments and delete them.
Now keep in mind, again, you are in the delete mode, once you hit DONE, the undo will undo the entire sequence of deletes you just did.
What if you just want to cut a section of a cline seg? No problem, in the delete mode and cline segs as the find filter, right click your mouse and select CUT from the pop up menu (do this before clicking on the cline segment).
Now pay attention to the command line: it will guide you with terse little comments like "pick first point".
CUT lets you remove a section of a segment between your two clicks.
Also, check youtube, there's a bunch of little 1-2 minute videos showing various commands.
Rotating parts? Super duper easy. Select the part with that four-way arrow thingy, click on the thing (remember the find filter!) and you right click and the options menu pops up.
I don't remember if ctrl-R is default for rotate, but if there is an alias defined for "Rotate" it will appear next to the word "rotate".
If it isn't, go to the Allegro command line, type alias, a space, press ctrl and R, (~R will appear) , a space and type rotate , then enter.
You have now defined ctrl R to be "rotate". So now if you're in the MOVE mode (four way arrow) , ctrl-r puts you in rotate mode.
Here you have many options, like the center point of the rotation, the rotation type, the angle, and how Allegro is supposed to handle connected clines.
Anyways, hope it was clear?