"I was able to reach that level [one of the top-rated juniors in the U.S.] primarily due to the vast training resources afforded by the Internet Chess Club" — Matthew Herman

You can have many different ratings on the ICC. There are ratings for bullet,
blitz, standard, 1-minute, and 5-minute, which are different speeds of
regular chess. There are ratings for bughouse, loser's, and wild, which
are for chess variants. See help definitions for an explanation of
what these mean. Type "finger" to see your current ratings.

To get a rating in one of these categories, you only need to play
a rated game. Do "set rated on" and then do "seek" to ask for a game.
You can ask for any time control or chess variant you wish. Type
help seeking for more information. Unrated games will not count for
your rating, but you are welcome to play unrated games.

Ratings usually range from 600 to 3000 on ICC, but there is theoretically
no limit at either end. You can type "best" to see the highest-rated players
on ICC. You can type "rank" to see where you stand among ICC members.
"statistics" shows the average ratings.

Ratings on ICC are similar to the USCF and FIDE, but are totally separate.
Do help survey for a statistical comparison.

A player's rating is "provisional" if he/she has played less than 20 games.
A rating is "established" if it is based on 20 or more games. A different
formula is used to calculate ratings for established and provisional players.
See help provisional for some information about provisional ratings.

Everything you wanted to know about rating formulas, but were afraid to ask:

The rating during the provisional period is the average of a set of values,
one for each game played. The value for a game against an established player
is the opponent's rating plus 400 for a win and minus 400 for a loss. For a
game against another provisional player, the value is moved towards the
previous average to lessen the impact of the unreliable result.
Extra points are then added to the rating for the purpose of keeping the
average rating of all established active players close to 1600. In particular,
1/5th of 1600 minus the current average is added to the rating.

To explain the established period requires the use of a formula. Suppose your
rating is r1, and the opponent's is r2. Let w be 1 if you win, .5 if you
draw, and 0 if you lose. After a game, your new rating will be:

1
r1 + K * [ w - ---------------------- ]
1 + 10 ^ ((r2-r1)/400)

I still need to explain the variable K. This is the largest change your
rating can experience as a result of the game. The value K=32 is always used
for established player versus established player. If you're playing a
provisional player, the factor K is scaled by n/21, where n is one plus the
number of games your opponent has played. See also help k-factor.

This formula has the property that if both players are established then the
sum of the rating changes is zero. It turns out that if the rating difference
is more than 719 points, then if the strong player wins, there is no change in
either rating.

Note that during the provisional period, BEATING a player whose rating is more
than 400 points below yours will DECREASE your rating. This is a consequence
of the averaging process. It's useful too, because it prevents the technique
of getting an inflated provisional rating after one game, and then beating 19
weak players to get an established rating that is too high.

Your initial rating that you gave at registration time is counted the same as
one draw against a player of that rating, brought to within the range 1000-2200.

Other info on ratings:

help FIDErating for the top 100 FIDE players from the latest rating list.
finger ROBOadmin for info on the searchable FIDE rating list on ICC.
help survey for a comparison of ICC, FIDE, and USCF ratings.