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?
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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.