How do I turn off anti-aliased brushes in Photoshop CS5?

You want to use the pencil tool rather than the brush tool to get rid of the grey shades around a line:

Anti-aliasing actually means you do get the slightly fuzzy edges. Aliasing means that either a black or white value is used as an alias for the true shape - the brush is circular but you can't represent a circle with square pixels. Anti-aliasing attempts to prevent the problem of using either black and white to represent an area the curve passes through by representing this area as a shade of grey.