A couple of key stats on each side of the ball:
Pitchers:
ERA is the Earned Run Average, and is the sum of earned runs (runs that do not occur as a result of walks or errors) divided by the total number of innings that the pitcher pitched.
WHIP: is Walks/Hits per Innings Pitched. Simply put, it is the sum of the walks given up and hits allowed over the total number of innings pitched.
Hitters:
Average: The number of hits divided by the number of plate appearances. When factoring the average, at-bats that end up in the hitter getting on base as a result of a walk or an error are omitted.
On-base percentage: This number is the percent of the time that the hitter gets on base by any means. Walks, being hit by a pitch, reaching on an error, etc. all raise this number.
Slugging %: This number is the sum of total bases reached by the hitter (single = 1, double = 2, triple = 3, home run = 4) divided by the total plate appearances.
OPS: Is the sum of a hitter's on-base percentage and slugging percentage. This is a pretty good indicator of offensive prowess.
There are many more, and I'll try to post some more later, but I have to go to class now...