Non-musicians can skip this post, though I'll try to make it as simple as I can, being as how Simple R Us ...
Minor pentatonic scales on the guitar involve various patterns, sometimes called boxes–the black dots indicate which string is pressed and which (relative) fret, and the "R" symbol stands for "root note."
These boxes can be moved up and down the neck to produce scales in different keys, and you can get by with only a couple of them. The first one I learned is, on this diagram, Pattern 5, then I got 1, and am current working on 4. In the key of A, for example, since they link together, you can go from 3rd position to 8th position starting with 4, then 1, then 2, and change octaves and tone and all, and depending on where you start, do the same positions in other keys. Easy, and would be easier still if the guitar's third and second string intervals didn't screw things up ...
Called pentatonic because they rely more on five notes instead of the regular seven in the major scale, if you use these right, you can solo over a blues or rock chord pattern and it will sound pretty good. Or at least it won't clash. Mostly this is because the half-steps are left out. This gives you the "bluesy" notes, upon which you can do things like hammer-ons or slides or vibrato, like that, just like B.B. King or Clapton or Stevie Ray.
Yeah. Just like them.
You can, and I'm trying to, learn the regular major scales for each key, and this gives you more options when doing other kinds of music leads, like the melody; however, you can get by with just the Em pentatonic moved up and down the neck.
Pentatonics are a good place to start if you want to take a solo when you are jamming with other players, and while I never wanted to make it cry or sing before, I'm finding that is something that now interests me.
And the beat goes on. I hope ...
P.S. You can go to YouTube and look for Blues Guitar Backing Tracks if you want to practice playing the lead. These are broken out into keys, and basically consist of a band playing chords and riffs, over which you can solo. So if you want to practice slow blues in A, you can go here; for a Texas shuffle in E, here, and so forth.
If you have a newer versions of GarageBand, there are a few rhythm tracks there, too, including both slow and faster blues. Look under "Magic GarageBand."