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
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
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
And the direction is, of course, (b/|b|).
This information was derived from my Materials Science and Engineering lecture notes.