Update field from other table

[Anna]

Hi,

I have two tables: both have the fields Code and Desc (but
the Desc values are different in each table).
How can I update table A's Desc field to table B's when
the codes are the same?
(i.e. if code a = code b , insert DescB value for code b
record in DescA field of code A record)

Hope this made sense!
Help gratefully received....

Thanks
Anna