cs vs last hit
The count versus the action. A last hit is the blow that finishes a minion; cs is the number of minions finished. You last hit to raise your cs.
cs
CS, creep score: the number of minions and monsters a player has killed. It is the counter of their income. "100 cs at 10" is a benchmark for good farm; "cs diff" is a reproach. "farm" means the same thing.
open the page →last hit
Last hitting is landing the final blow on a minion, the only one that gives gold. It is a technical skill, and the hardest one to learn at first: hit too early and you get nothing, too late and the same. "just last hit" means only take that blow, so the wave does not push.
open the page →Why they get mixed up
Both are about minions, and both show up in the same sentences. But "cs" is a result, "last hit" is how you get it. A player can have a good cs by pushing everything; a player who last hits cleanly also controls where the wave sits. The second is worth more than the first.