USB 2 and USB 3 are the same size.. USB 3 can carry more current (900ma vs 500ma) and more data, obviously. USB3 is also blue as opposed to black on the inside.
There are also 2 different types of cables - USB Data/Charge cables (also know as Sync/Charge), and USB Charge cables. Obviously the USB Data/Charge cables do both, but, might be limited to just the 900ma of power. These are the cables used to plug say, a phone, into your PC. The USB Charge cables are designed for the 'wall wart' type of power supplies and can usually carry more power, like 2.1a for charging tablets, and dont carry data (sync).
If you are buying a a new cable AND need it to carry data, get USB3. Otherwise, all you need is a 'USB Charge Cable' which will allow things to charge, but carries no data.
As for the coupler you posted, yes, that will do to connect 2 USB2 or USB3 cables, or a combination of both. How much power it will carry I dont know, but I would assume it is for Data and thus not have a very high amp carrying capacity.