DEPC is an acronym for di-ethyl-propyl carbonate. It is typically used to treat water to remove RNases for use with RNA.
DEPC reacts with amine, hydroxy and thiol groups of proteins (such as RNAses) and inactivates RNAses. Treatment involves adding DEPC to 0.1% v/v and incubating at 37°C for 1 hour to overnight followed by autoclaving. Autoclaving destroys DEPC and is an essential step. Esters may be generated during autoclaving giving rise to a 'fruity' smell (that is not coming directly from DEPC).
Note that DEPC cannot be used with chemical solutions that have amine groups, such as Tris and HEPES buffers, or mercaptans. In such cases, use DEPC-treated water to generate the solution.
Di-methyl-propyl carbonate (DMPC), a safer alternative to DEPC (known carcinogen), is used in exactly the same way.