I think that Ashcroft and Mermin (Solid State Physics) would give you a decent explanation as well.
Bassically, you can have a slip fault along any of the preferential crystal directions, so if you slip a single plane over, the next plane can rebond, in this manner a linear dislocation can move through the material. Any type of discontinuity in the crystal, be it an ampty lattice site, an inerstical attom, an impurity attom etc, causes the energy required for the slip to be greater, thus the material is stronger.
Sorry not a meturlagist, just a lowly Physicist. (Which means I need drawing to explain anything well)