In addition to crochet abbreviations, crochet symbols are often used to help the reader visualize a pattern's instructions. The following is a glossary of common crochet symbols, in "E2" form (due to ASCII character set limitations) and a verbal description of each symbol. These symbols are accepted as modern standard crochet symbols internationally, however the E2-ified symbols were created by me to make pattern illustration on E2 possible.
o - A hollow oval.
Double crochet
T\ - A capital T with one diagonal slash through it (each slash represents one yarn over before working the stich).
Double triple crochet
T\\\ - A capital T with three diagonal slashes through it.
Half double crochet
T - A capital T.
Single crochet
+ or X - Either a short vertical stick crossed in the middle, or (less commonly), a capital X.
Slip loop
@ - A solid colored oval. NOTE: The slip loop symbol is often left out in diagrams because its use is generally implied in every crochet work.
Slip stitch
n - A simple arch.
Triple crochet
T\\ - A capital T with two diagonal slashes through it.
Turning chain
% - No comparable modern standard symbol. NOTE: This symbol can represent any number of stitches, however many are appropriate for the turning chain where it is used, unless otherwise noted by the pattern's author. Therefore, in a work of single crochets, it is 1 chain, while in a work of triple crochets it can be 4 or 5 chains.

For example, a work of 5 rows of double crochets could be illustrated like this:

  T\ T\ T\ T\ T\ %
  %  T\ T\ T\ T\ T\
  T\ T\ T\ T\ T\ %
  %  T\ T\ T\ T\ T\
  T\ T\ T\ T\ T\ %
@ o  o  o  o  o  o

Source: Crocheting in Plain English by Maggie Righetti