I 'm sure you can look up "stepper motor" on the internet and get a better and more in depth definition than we can provide here. What it seems you are asking is what do you need for gear to fit a specific application. As in any practical engineering question, the specifics of what you want to do will decide what you need to achieve it.
A lever moving back and forth a set amount every minute is so vague that there is no answer. How much force is required? How much distance must it move? Is the movement periodic, uninterrupted, and independent of circumstance?
The answer could easily be that you don't need a stepper motor and computer and drive circuitry at all, you need a motor and a timer, or a solenoid and a timer, or even just a synchronous motor and linkage. Any of the above could be the size of a few gram pager motor, or weigh hundreds of kg. depending on what lever you're talking about.
In other words, it's important to have clear in mind what you want to do first in specifics, before researching the means to do it. If you start out just choosing a means, you'll make finding a good solution hard for yourself. The great attraction of the internet is an exposure to advances in affordable cutting edge digital technology. The big problem is thinking therefore that it is the the only way, or even the best way to achieve a simple result.
If you said for instance,I'd like to wave the wing of a copper garden dragon weighing 2 pounds about 3 inches once a minute, somebody here might say, try this timer and this windshield wiper motor and this speed reducer, etc...