http://www.stathead.com/bbeng/woolner/statglossary.htm

Statistics Glossary
by Keith Woolner

Defensive Statistics
DA (Defensive Average) 
The Baseball Workhop/Project Scoresheet has been tracking the location of
every batted ball in every game for the past several years"Defensive
Average", or DA, is the rate at which fielders turn balls hit into their
zones into outs. DA is analagous to a fielder's "batting average against" in
that it measures times reached base (by hit or error) per opportunity (ball
hit into his zone). The areas of responsibility for DA span the entire field
-- no portion of the field in considered to be beyond the reach of some
fielder.  More info on DA is available in the Defense section of the Stathead
Baseball Engineering Library.

ZR (Zone Rating) 
STATS devised their own system of zones to track locations of batted balls.
STATS uses this data to measure a fielder's "Zone Rating", or ZR. Zone Rating
differs from DA in several ways, notably in that an outfielder is credited
with two outs on a double play (DA counts just the fact that a batted ball
was converted into an out, without attempting to measure the value of the 2nd
out). The areas of responsibility for ZR are smaller for each fielder than in
DA. ZR areas of responsibility do not span the entire field -- some areas
(for example, deep in the gap between CF and RF) are considered to be a "no
man's land" that is ordinarily beyond the reach of fielders, and thus a ball
hit there is not considered an opportunity.

RF 
Range Factor, RF = (PO + A) / Innings Played * 9 (sometimes computed as RF =
(PO+A)/Games Played) Range Factor is, essentially, the number of plays made
per 9 innings (or game).  It is a somewhat better measure of defense than
Fielding Percentage, in that it does take into account how well a fielder
gets to a batted ball.  RF is calculable for most of baseball history, and as
such is probably the best tool we have for assessing historical defensive
performance.  RF's are generally only comparable at particular positions
(i.e. you can compare a 3B to another 3B, but not to a SS or a CF).   RF is
sometimes adjusted for the number of strikeouts the pitching staff makes in
front of the fielders (more strikeouts means fewer outs made by the
fielders).  The major disadvantages of RF are that there is no real control
for opportunities (you can't tell whether, say, a shorstop's RF is high
because the he was an excellent fielder, or because he played behind a
extreme groundball pitching staff).

Fielding Percentage (aka Fielding Average) 
Defined as F% = (PO+A)/(PO+A+E).  PO = Putouts, A = Assists, E = Errors.
Fielding percentage is the most common defensive rating in baseball today,
which is unfortunate because it is probably the least accurate of any major
commonly used statistic in baseball.  Why?  It doesn't measure what it claims
to measure (success rate of fielding opportunities).  If you accept that the
primary role of a fielder is to convert batted balls into outs, then fielding
percentage measure only those batted balls which the fielder could easily
each, and only those failures to convert them that the official scorer deemed
should have been made with ordinary effort.  A lack of range or mobility does
not show up in fielding percentage.  Indeed a fielder who does not attempt to
make a difficult play fares better than an Ozzie Smith-type who can get to
the ball, but may get charged with an error in trying to complete the play.
 The difference in errors among major league players in the modern era is so
low (amounting to a handful of balls per year), that differences in range
dominate the true differences in defensive performance, yet fielding
percentage essentially ignores it.  The keys here is that opportunities are
not independent of the fielder's own performance, and that the most frequent
kind of failure is not penalized. Avoid using fielding percentage if at all
possible.

Fielding Runs 
Fielding Runs (or FR) are a generic term for any statistical treatment of
fielding that converts a fielder's performance to runs.  The most common
approach is that used by Total Baseball.  A full description of the technique
can be found in the glossary on the Total Baseball web site.  Basically, TB's
approach involves weighting the number of putouts, assists, and double plays
made by each fielder, and comparing the totals to positional norms to come
out with a figure above/below average.  Each extra out made (or hit allowed)
is worth X runs in the LWTS model, which leads to the FR figure.  The
disadvantages of the TB method are similar to those of Range Factor, and the
FR figures are considered particularly unreliable for catchers and first
basemen.  Career figures are also considered better indicators than
individual seasons.

There are also FR figures based on Defensive Average, which are considered
more reliable in the few years where DA is available.  In principle, FR could
be done for ZR also, but I am not aware of any published attempts at doing
so.