Hello everyone. I'm familiar with Foobar2000 and I have been curious about replaygain. The question I have is, how do I apply replaygain to all my tracks so that I don't have to fiddle with the volume knob with every track? I don't understand the difference between applying by individual tracks and by albums. Could someone chime in on how I would do this without causing the peaks to clip? 

Track Gain: increases or decreases the volume of that track to on average 89 dB. All tracks will be at about the same loudness.
Album Gain: increases or decreases the volume of the album to be on average 89 dB. This allows quiet tracks on the album to remain quieter than the loud tracks on the album.

Basically if you listen to an entire album at a time, you should use 'Album Gain'. If you listen to shuffled/random tracks, you should use 'Track Gain'.

If your albums are arranged by folder then you first want to go to Preferences -> Context Menu -> ReplayGain and select "Scan selection as albums (by folders)". That way if you scan two albums with the same metadata but in different folders they will be treated as from different albums. Look at the top of your library, right click 'All Music', select ReplayGain and then 'Scan selection as albums (by folders)'. Scanning gives both track and album gain. If your music isn't organized by folders you can just scan by tags.

Once that is done and you update the tags (the whole process can take awhile with a big music collection) then go to Preferences -> Playback and under ReplayGain select if you want track or album gain and then use 'apply gain' under Processing. You could use 'apply gain and prevent clipping according to peak' to prevent clipping but I use the Advanced Limiter DSP (Preferences -> Playback -> DSP Manager). Don't think there would be a difference either way just that the Advanced Limiter will work on tracks without ReplayGain.

All the ReplayGain data is stored in tags and can be removed or just turned off in foobar.

Thank you very much. Since I do have all my music in folders, I guess I will use the "by folders" option. One more quick question. If I scan all the tracks by folder, will it also scan with a separate value per track, just in case I want to mix and match my music?

Track gain is always given when scanning and is the same regardless of how you scan.