Camera Progression

Discarding the rather confusing Japanese and American model nomenclature for the entry level series, Canon's model numbering system is fairly simple. The more digits before the 'D', the lower the camera tier. Except for the single-digit tier, larger number represents a later model.

A useful link: Canon timeline
http://en.wikipedia.org/wiki/Template:Canon_DSLR_cameras

Four digit series: 1000D, 1100D are the least featured, lowest priced, introduced not too long ago. Fairly popular, but most DSLR buyers are after something a bit more advanced, so Canon does not update the budget line too often, hence it goes up by 100

Three digit series: 300D, 350D . . . . 650D is the Canon's extremely popular entry-level tier, probably generating more sales than others combined. It is most often updated - roughly every year - year and a half, and goes up by 50. Not certain what Canon will do when it gets to 950D in 6-8 years, but entire photography industry might be very different by then.

Two digit series (20D, 60D etc) are the enthusiast level, usually sharing same sensor as 3-digit tier, but offering more advanced controls, more robust bodies, pentaprism viewfinders, deeper burst buffers and other features. Updated roughly every 2 years or so.

In single digits, each number represents its own tier, and lower numbers represent higher tier. Within these, later models bear 'Mark' denomination. I.e. the successor to 5D is not a 6D, but '5D Mark II', 5D Mark III etc.

1D series is the professional top line, and has historically included two variants: a very fast, but not quite full-frame 1D, and a full frame, but not as fast, 1Ds. 1D went up to Mark IV, and 1Ds went up to Mark III. The reason for separation was that camera processors at the time lacked the power to handle 8+fps at full frame.

Rather than releasing the next 'Mark' of either, Canon has now combined the two into one line: 1Dx is a full-frame pro camera with enough processing power to handle 12fps.

The announced but not yet released 1Dc is basically 1Dx redesigned towards serious cinematography, and among other things, can capture 4K video.