Ambassador-class
The Ambassador-class is a large starship type equipped for scientific missions as well as for combat.  The class appears to be an intermediate design between the Excelsior and Galaxy classes.  Introduced in the early 24th century, the Ambassador is now among the aging ship designs.

The first appearance of an Ambassador-class starship, namely the Enterprise-C, was in the form of a relief on the "history wall" in the Enterprise-D's observation lounge in the first TNG season.  This half-model did not show many details, but it had an almond-shaped engineering hull akin to the Galaxy-class Enterprise-D, as opposed to the later tubular hull.  Production designer Andre Probert also created a painting of this early version.  The book describes this painting as an early design stage of teh Enterprise-D, so it would predate the TV series by at least several months.  Probert, however, states that this design was supposed to represent another ship the Enterprise-D was to meet with and that he intentionally "layed out its lines to fit between those of the 'B' Enterprise and our 'D' ships so that it would feel like a logical design progression".

The early version is not exactly canon, since the painting on one hand was never shown on screen, while the models on the "history wall" are not very precise.  The Enterprise-C relief can be regarded as an artistic rendition falsifying the ship's real look, the same applies to the Enterprise-B which was shown as late as in "Star Trek: Generations", but with additional hull extensions combared to the standard Excelsior on the wall.

Apart from the early version, there are two variants of the Ambassador-class.  The first one was built to represent the USS Enterprise NCC 1701-C in TNG: "Yesterday's Enterprise".  Her design deviates from the relief on the wall in that the engineering hull is now tubular.  The second one was used for the USS Zhukov (TNG: "Data's Day") and USS Yamaguchi (DS9: "Emissary").  The Ambassador-class USS Excaibur could be briefly seen in TNG: "Redemption II" and is probably a modified version too.

The two variants are subsquently referred to as "Enterprise-C" and "Yamaguchi".  Looking at the studio models, there are nine major and a few minor differences:
1.) Bridge Module - The 12 normal-sized lifeboats on deck 3 of the Enterprise-C were replaced with 20 small rectangles and two large shapes, probably transporter emitter pads, one on either side.  The 16 small rectangles on deck 2 were removed, and the number of lifeboats was reduced from 4 to 2.  The Yamaguchi has 8 additional lifeboats behind the bridge module, compensating for the missing lifeboats on decks 2 and 3.
2.) Sensor Dome - The Enterprise-C has a clear round sensor dome directly underneath the saucer hull, while the Yamaguchi's sensor dome is located underneath an additional platform.  This platform is similar to the Excelsior's, with rectangular extensions pointing in six directions, separated by angles of 60 degrees.
3.) Deflector Dish - The Enterprise-C has a main deflector akin to the Constitution refit, showing the reddish color of the resin with 16 radial white/gray lines.  The dish shines blueish when the model is lighted.  The Yamaguchi's deflector dish is painted mid blue, and has an additional rust red or orange center part.
4.) Saucer and Neck Location - The Enterprise-C's neck is sticking out roughly 6m behind the backward saucer rim, while the Yamaguchi's neck is recessed by about 6m.  The neck part and its location on the engineering hull appears to be the same for both models, so the complete saucer was moved about 12m backward for the Yamaguchi, and either a part of the curved saucer bottom or the neck was shaved off to make it fit again.
5.)Nacelle Locations on Pylons - The distance between the nacelle front tips and the saucer is roughly the smae for both version, maybe only a bit closer for the Ymaguchi.  It seems the forward shift of the secondary hull is compensated by moving the nacelles of the Yamaguchi back, away from the saucer rim.  This is corroborated by the bottom view of the ship which shows that the pylons are closer to the forward end of the "bulge" on the nacelle bottom.  Therefore the overall length of about 526m according to the Star Trek Encyclopedia II remains about the same also for the Yamaguchi version, or the ship is only insignificantly shorter.  The renderings assume the two ships are of the same length.
6.) Bussard Collector - The Bussard collectar was covered with an upper and lower cap for the Yamaguchi version.
7.) Third Shuttlebay - The Enterprise-C has two shuttlebays-one on deck3 and on in the stern.  The Yamaguchi has an additional underslung shuttlebay beneath the engineering hull undercut which resembles an Excelsior shuttlebay turned upside down.
8.) Engineering Hull Width - The Enterprise-C's engineering hull top side is strongly conical, narrowing at the rear end to only one third of its front end width.  The Yamaguchi's engineering hull is much less conical and therefore considerably wider at its read end.  This is one reason why the platform around the Enterprise-C appears to be much wider than that of the Yamaguchi.  Still, it looks as if the shape of the platform has been modified too, along with the 2x4 raised panels on the nacelle struts on each side which seem to have been moved farther away from the ship's centerline.
9.) Impulse Engine Color - A difference that can be identified in VFX shots rather than on photos of the studio miniatures is the color of the impulse engines.  The plasma glows blue on the Enterprise-C, while it has the usual red, LED-like look on the Yamaguchi.

There are a few minor modifications, for instance additional transporter emitter pads behind the bridge module and emitter pads and lifeboats on the pylons on either side of the engineering hull.  The Zhukov looks essentially identical to the Yamaguchi, except for the hull markings.  The name, however, should read "Zhukov" instead of "Zuhkov".  The hull number "NCC-62136" is another mistake.  It doesn't fit into the normal range of Ambassador-class registries and was probably supposed to be "NCC-26136".