The Android Dictionary (aka "What the hell are they talking about?!?!")
If you're new to Android (and even if you aren't), you'll quickly discover that there are a ton of terms to keep up with. And to that end we've worked up the official Android Central Android Dictionary. Pretty self-explanatory, eh?
You can check out the Android Dictionary here.
That said, we're bound to be missing something. If so, sing out in this thread and we'll get it added.
Bootloader and apk info...
A bootloader is a small program that runs before any other, and lets you choose which operating system you'd like to run (if you have more than one installed), or (for example) a maintenance program (like recovery mode).
Google LILO (LInux LOader) or GRUB (Grand Unified Boot Loader) for more detail (if you like...). I use GRUB on my Linux box at home, to choose between Ubuntu (32 or 64-bit versions), and Vista (64-bit).
It's definitely not a "mode".
I haven't researched it, but I'd believe apk stands for Android PacKage, which would consist of the app, install and/or remove scripts, and configuration files (some of these may be optional, though not the app <grin>).
They'd be analogous to .deb and .rpm files on Debian and Red Hat GNU/Linux based systems.