The basic set is the 208 as folks have mentioned above. Note that the different pictures are distinct cards, not variations as such. “Collins Open Mouth” and “Collins Closed Mouth” are just different cards, like “Cobb Bat On” and “Cobb Bat Off” for T206s.
The “Master Set” is really whichever variations you decide you want to collect. The PSA and SGC registries have their own opinions about which variations to include. I wouldn’t consider those “official” outside of the context of the specific registry. Some people choose one of those to go by, especially if they have a set registered. Both are slow to add new variations, and I’d suggest only using them as a starting point and adding whatever variations you find interesting even if the registries don’t list them. “Unlisted” variations tend to be less expensive than “official” ones, since registry folks are chasing the latter.
Probably the best way to learn about “unlisted” variations is this board. If you search in Google for
site:net54baseball.com new t205 variation
that alone gives a wealth of information from other board members.
A few highlights, some already mentioned in Cozumeleno’s link above:
The Collins Closed Mouth “Yellow Elephant”:
http://www.net54baseball.com/showthread.php?t=150383
A missing decimal point for Sovereign Fred Snodgrass:
http://www.net54baseball.com/showthread.php?t=218157
Printing differences for Hoffman, including the “green stripe”:
http://www.net54baseball.com/showthread.php?t=192728
A few Titus cards have a blotch over the world “has”:
http://www.net54baseball.com/showthread.php?t=243236
Some Honest Long Cut Latham cards seem to have remnants of the “W.” before the “A. Latham” on the back.
http://www.net54baseball.com/showthread.php?t=240610
Doc White missing quotes on Polar Bear backs
http://www.net54baseball.com/showthread.php?t=140229
and a few others.
Individual collectors have different opinions on what “should” count as a variation versus a print defect, so keep that in mind when reading these threads. But it’s really up to the individual collector, what they want to collect!