Differences in varieties of 2x2 tile ( 3068 3068a 3068b )?
The difference is actually explained within the naming, which comes from Bricklink, a place where people buy and sell LEGO items around the world. In cases like Bricklink, especially with AFOLs, it is necessary to know which exactly parts you are dealing with.

During the years LEGO has been upgrading parts, where Tile 2 x 2 is not an exception. 

Initial version is 3068a, which comes without a groove - its side is same like any other plate - smooth. While it may look great in an assembly it is hard to remove - there is no real gap between a tile and other part it is attached to. There's also no stud on top you could somehow use to take things apart (for example, with brick separator).

So LEGO eventually updated this tile (Bricklink denotes it as 3068b) to include a small groove, which helps with disassembling tile from other bits without much effort.

Bricklink uses "Undetermined version" for cases where seller can list an item without specifying which actual type (with or without a groove) it is.

I don't have any old parts on hand to take a picture, however you can see both tiles used in digital form are different. Green one doesn't have groove and Yellow one has.