ARP measures the amount of runs that a relief pitcher prevented from scoring above what an average relief pitcher would have prevented. ARP is adjusted for the situation in which the pitcher was used.
ISO Isolated Power
ISO is the ratio of extra bases that a player has accumulated to the number of at bats he has received. ISO is essentially a player's SLG minus his batting average. This has the effect of giving a player credit only for extra base hits. ISO is not a useful measure of player value on its own, but is a very effective measure of a player's extra base ability.
OBP On Base Percentage
OBP is the ratio of the number of times a player reached base safely to the number of opportunities he had to reach base. It effectively measures a player's skill at not making outs. Since outs are a teams most precious commodity, OBP measures perhaps the most valuable and fundamental skill a player can have.
OPS On Base Plus Slugging Percentage
OPS is a crude metric that simply sums a player's on base and slugging percentages. It is probably the most popular non-traditional measure of overall batting performance due to its simplicity. However, it has drawn criticism from performance analysts for its inaccuracy relative to other advanced metrics and because it works by adding two numbers with different denominators together to produce a conceptually meaningless quantity. It is best used as a quick and dirty estimator of batting prowess.
SLG Slugging Percentage
SLG is the ratio of total bases that a player has accumulated to the number of at bats he has received. It is essentially a weighted batting average that gives a player more credit for extra base hits.
UZR Ultimate Zone Rating
UZR is a defensive metric that uses play-by-play data to determine how good a player's defense is. On Fangraphs, it is denominated in runs saved above average.
VORP measures the amount of runs that a player contributed above what a "replacement player" at the same position would produce. VORP considers only offensive contributions.
WARP Wins Above Replacement Player
WARP measures the amount of wins that a player contributed above what a "replacement player" at the same position would produce. WARP considers both offensive and defensive contributions.
WXRL Win Expectancy added above Replacement adjusted for Lineup
WXRL measures the amount of wins that a relief pitcher contributed above what a "replacement player" would produce. WXRL differs from WARP because it is adjusted for both the game situation in which the pitcher was used and the hitters that the pitcher faced.