OPS is the sum of a player's on-base percentage and his slugging percentage. The first measures how often the player makes it to first base, while the second gives analysts a way to measure how often he hits a double, triple or home run. To figure the player's OBP, divide the total number of hits, walks, and times hits by a pitch by the number of times at bat plus walks, sacrifice flies and times hit by a pitch. SLG is calculated by dividing the total number of bases reached for all base hits (a double counts as 2, for example) by the total number of times at bat. Once you've figured OBP and SLG, you can figure the OPS. For example, a player with an OBP of .280 and a SLG of .500 will have an OPS of .780.
Some aren't terribly fond of OPS, because the measurement treats on-base percentage as equal in importance to slugging percentage, despite the fact that OBP has been demonstrated to be nearly twice as important in scoring runs as SLG. OPS Plus "normalizes" a player's OPS by accounting for variables like league and park effects.