It is defined as "A
vector that denotes the
magnitude and direction of
lattice distortion associated with a
dislocation." (Callister,
Materials Science and Engineering an Introduction, 776)
The Burgers vector can be used to identify the nature of a dislocation in a crystalline structure. If the Burgers vector is parallel to the dislocation line, this indicates a screw dislocation. If, however, it is perpendicular, this indicates an edge dislocation. An angle other than these (other than 0, 90, 180, 270 degrees) is indicative of a mixed dislocation.
For a given dislocation line, which may change direction and nature in one crystalline lattice, the Burgers vector will remain the same. In metallic materials, the direction will be in a close-packed crystallographic direction, with magnitude equivalent to the interatomic spacing.
In face centered cubic and base centered cubic structures, the Burgers vector may be expressed as
b=(a/2)[hkl]
where a is the unit cell edge length and [hkl] is the crystallographic direction having the greatest linear atomic density. As some basic vector math will tell you, the magnitude of the vector is
|b|=(a/2)(h2+k2+l2)1/2
And the direction is, of course, (b/|b|).
This information was derived from my Materials Science and Engineering lecture notes.