Now, a thing can be thought to be good -- which is to say that we can desire it -- in three ways: it can be desired for its own sake (this is called a "virtuous good"); it can be desired for the sake of enjoying it (a "pleasurable good"); or it can be desired for the sake of some other good (a "useful good").
In ST I-II,8,2, St. Thomas teaches that the will is concerned both with the end and with the means -- that is to say, both with virtuous and pleasurable goods, which are ends, and with useful goods, which are means. We desire both means and ends with our will.
That said, desiring an end is the fundamental act of the will, while we only desire means because they relate to ends we desire. So it makes sense that we use the verb "to will" primarily for the act of desiring an end, and a different verb -- St. Thomas uses "electio," which can be translated "to choose" -- for the act of desiring a means to an end.
And, to keep things from being too simple, the "simple act of willing," which is the act of desiring an end, which is the act of desiring some virtuous or pleasurable good, is sometimes called "volition."*
I think my convention will be this: If I use the verb "to will" without saying what exactly is being willed, then I mean the simple act of willing an end. But I will also sometimes write "willing the means," meaning the act of desiring a means to an end. (This may be a case where everything makes perfect sense until you try to explain it.)
Article 3 ties it all together: Since acts are specified by their objects, and since the end is a different species of good than the means, it follows that the act of willing a certain end and the act of willing the means to that end are two different acts. They both move the will toward the same end, but the first act moves the will "absolutely" or directly to the end, while the second act moves the will to the end as the reason for willing the means.
By the way, I called the act of willing a certain end "the first act" in that last sentence because it was the first of two acts mentioned in the sentence before. But it's also the first act logically -- you can will the end without willing the means, but you can't will the means without willing the end -- and, often enough if not always, it's first chronologically -- you will the end first, then settle on the means, then will the means.
STILL TO COME: That second act of willing the means will turn out to be the act of intention.
AND MORE: One wrinkle in the above is lumping pleasurable goods in with virtuous goods as ends. Virtuous goods are desired for their own sake, so it makes sense to call them ends. Pleasurable goods, though, are desired for enjoyment, so doesn't that make a pleasurable good a means to the end of enjoyment? We'll iron this out, I hope, when we get to the question on the act of enjoyment.