Open Type comes in two "flavors" True type and Post script. From what little I understand Open Type was built around the true type format, but expanded upon.

OpenType is an extension of TrueType. One of the ways it was extended was to allow PostScript outlines instead of TrueType outlines. So, OpenType-TT contains TrueType outline data, and OpenType-PS contains PostScript outline data. Either "flavor" will work well for print. TrueType outline-based fonts tend to be better for on-screen applications on Windows. They work equally well on Macs. OpenType-TT fonts don't work at all on pre-OS X Macs.

Thanks! This is very helpful. Would I even notice a difference between the two when printed?

Did you ever see a difference?

Truetype fonts got a bad rep because of loads of cheapskate 'free' fonts, which looked bad on screen and worse in print. (And I wonder: could this be because the first widely available 'font design' tools were TTF, or Windows based?)
Technically, however, they are equivalent to Postscript fonts. There is even an ongoing debate about that -- the mathematical curves (splines) in TT fonts are different from PS Bezier curves, and some maintain one produces "nicer" curves than the other. I wouldn't know.

TTs may look better on screen, because its format has extensive support for screen optimization built in, especially in smaller sizes. But it's a lot of work for the designer to program all that data; and the auto-hinting of screen drawing programs gets better by the year, and screen rez gets higher and higher every year. Besides, it really doesn't matter for printing.

There is another sort of self-imposed limitation in PS-flavoured fonts: the Design Grid is by default 1000 x 1000 integer units, while for TT-flavoured fonts it's 2048 x 2048. That means that (again, by default!) you can have twice as small elements in TT fonts as in PS. However, I believe there is no longer a real technical reason for this -- I'm sure the Font Creators here can fill in the details.

Would I even notice a difference between the two when printed?

For the same font in one format or the other, for print, probably not.

> could this be because the first widely available 'font design' tools were TTF, or Windows based?

Way back in the early 1990s when the first cheap "1.000 fonts" CDs came up, the copyright line usually said "Converted by Alltype". In fact, what damaged the reputation of the TT font format was a flood of bad quality automatically converted PS (Type 1) fonts, done with tools like Alltype or Fontmonger. The font creation programs then were Fontographer and Type Designer, both working internally with Type-1 curves. Very few people really made fonts with gimmicks like "Softy".

Andreas