That's a general question.. so here's a general answer:

-Grignard reagents are very strong nucleophiles, but also very strong bases.
So you can't get them wet. Adding water or an alcohol to a Grignard will result in an acid-base reaction:
CH3MgBr + H2O -> CH4 + MgBrOH

-Because Grignards are such strong bases, they don't react with alkyl halides like SN2 nucleophiles do. Instead, they will do E2 reaction.
Example:
C3H7Br + NaCN -> C3H7CN + NaBr (SN2 Reaction)
C3H7Br + CH3MgBr -> C3H6 + MgBr2 + CH4 (E2 elimination. The SN2 product C4H10 is NOT formed!)

-Grignards usually react with carbonyls, such as aldehydes, ketones, and esters.
With esters, they react twice.

See MendelSet 1324 for a bunch of Grignard related questions.