Single
The most common hit in baseball and the one whose value depends most on who is on base.
What it actually is. A single is a hit on which the batter reaches first base and stops there, with no error and no fielder's choice involved. It is the most common kind of hit by a wide margin. A runner on second usually scores on one; a runner on first usually reaches third only if the ball is hit to right field or gets through cleanly.
What somebody who knows the game gets out of it. A single is worth about half a run on average, but that average hides everything interesting. With the bases empty it is worth a fraction of that; with two out and men on second and third it is worth most of two runs. This is exactly why run expectancy exists and why a hitter's RBI total tells you about his teammates. A ground ball single and a line drive single are also different events going forward: the first is roughly a coin flip that the defense lost, the second is repeatable contact quality.
The numbers worth knowing. Roughly two thirds of all hits are singles. A league batting average around .245 means the average hitter reaches on a single in a bit under a sixth of his at bats. Linear weights put a single at about .47 runs.
What people get wrong about it. Reaching first on an error is not a single, and reaching on a fielder's choice is not either. Both look identical from the stands and both leave the hitter standing on first, but only one of them helps his batting average, which is one of the reasons average is a noisier number than it looks.
At Little League and travel ball. A very large share of youth singles are balls the defense failed to convert, not balls that were struck well. The read on a youth hitter is not his hit total but where the ball went and how hard: a hard ground ball at a 10U shortstop is a better sign than a soft flare over him, even though the scorebook says the opposite.
Batter's Eye: look up anything in baseball