Normalizing raises the volume level on a song to some user selectable percentage of maximum (before clipping starts).
Dithering paradoxically applies very low level noise to the file...smoothing the waveform. Better sound results. The factory CD's you listen to have been dithered. Ripping maintains the dither but normalizing introduces round off errors (un-dithers), so you generally need to re-apply the dither.
Ogg is a newer format. Better sound, better tagging, gapless playback (don't care too much about this, I let the playback engine do crossfades), etc.
Check out the net for more in-depth coverage on all of this. Hydrogen Audio is chock full of info.
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Replay Gain tags the file with a decibel value (to raise or lower). It relies on the player to raise the volume (think of it as a pre-amp) to the tagged values. It is lossless (no rounding errors introduced) but relies on player to have RG capability (and the same type of RG, for that matter). Many don't...and you would be VERY hard pressed to hear a difference in playback quality between an RG'd file and a Normalized/Dithered file. N/D files will be volume level adjusted permanently, so they work on EVERYTHING.