Compact Disc technology became commercially available in 1983, in time for Let's Dance, his first album under his new contract with EMI. The next year, Tonight was also granted a CD release. At this point, his former record company, RCA, who held the rights of his back catalogue, decided to make available most of his previous albums on CD, too. David Live was left out for some inexplicable reason, as was his self-titled debut, since it was released by yet another company, Decca.
However, the RCA CDs were very poorly mastered and were discontinued after only a few years. Thus there was a time when Bowie CDs were diffucult to pick up, unless you were after a new release. Then, in 1990, under circumstances beyond my scant knowledge, the material formerly under the control of RCA fell to EMI and Ryko, the latter company insisting that as well as remastering the entire catalogue, bonus tracks would be required on every album (Aladdin Sane was the exception because of a few mixups with tracklistings). So not only could a whole new generation of Bowie fans pick up CDs with excellent sound quality, they were also treated to extras that not even the older fans would have heard before.
In the mid90s (somewhere between 1995 and 1997), all of the albums with original releases in the 80s (Let's Dance to Tin Machine) were given rereleases by EMI and Virgin with bonus tracks. However, although Never Let Me Down gained three extra tracks at this time, it also lost Too Dizzy, meaning that there isn't a single version you can get which has all of the tracks. At the same time, 1. Outside was also given a special release in some countries with either one bonus track, Hallo Spaceboy (PSB Mix), or an extra CD (which I think was made available in Australia only). Some versions also dropped Wishful Beginnings, while Japan also had Get Real included in the middle of the CD.
Fortunately, things get a lot simpler from here. In 1999, EMI released another round of rereleases, called the David Bowie Series, with 24-bit remastering but alos with all of the bonus tracks deleted. However, this series stopped at Tin Machine and did not include any live albums, although this is being rectified with the recent rerelease of Ziggy Stardust: The Motion Picture and plans for Santa Monica '72. Further, every album from Black Tie White Noise to 'Hours...', except The Buddha Of Suburbia have been released again this year. As far as I know, along with TBOS (arguably the American release was simply delayed, not a proper rerelease), Tin Machine II is the only album not to have been given the rerelease treatment.
It is somewhat confusing and I'm not sure if I've made everything quite as clear as possible - and undoubtedly there are some mistakes in there somewhere - but it should give you the general idea.
Who sucks you while you're sleeping