The name Shabbat Shuva is taken from the opening words of this week's haftora, "Shuva Yisrael - Return, O Israel." The second name, Shabbat Teshuva, is derived from the fact that this Shabbat falls out in the middle of the Aseret Yemei Teshuva, the Ten Days of Repentance. This name is also associated with the haftora, as its central theme is returning to G-d.
The two names of this Shabbat reveal a timely lesson. The phrase "Shuva - Return" is grammatically a command. G-d commands us to return to Him in repentance. Teshuva, by contrast, is a noun denoting the action itself, the actual return to G-d.
The word "Shuva" relates more to the One who issues the command than the person being addressed. It implies a situation in which the command has already been issued, but not yet carried out. The command itself imparts a certain measure of strength, but does not ensure that it will necessarily be fulfilled in the future. "Teshuva," on the other hand, implies that the action has already been taken, i.e., that the teshuva has already been done. But if that were the case, why would we continue to refer to this Shabbat as Shabbat Teshuva?
The answer is that the act of teshuva consists of both the command to return to G-d and its subsequent implementation.
Shuva teaches us that even after a Jew has done teshuva, he still needs to work on himself. No matter how much teshuva a person has done, it is always possible to rise higher; hence the directive, "Return, O Israel unto the L-rd your G-d." Thus it is understood that there is always room for improvement, for an even deeper and more infinite level of teshuva, as G-d Himself is Infinite and without limitations.
This, then, is the lesson of Shabbat Shuva: A Jew must never content himself with whatever spiritual accomplishments he has already attained. He must never think that, because he has worked on himself a whole week, he is now entitled to "rest" because it is Shabbat. No, today is Shabbat Shuva! Even after one has done teshuva, more work is required. For the service of teshuva is continual and without end.