exactly like the definition if each word

parallell, 2 turbos the same size where the exhaust mani splits and enters each one equally. compressor outlets merge together into one charge pipe.

sequential, 2 turbos of different size, smaller one helps spool the larger one. exhaust enters the smaller turbo, the DP then enters the bigger turbo's turbine inlet, then exits into the exhaust. compressor outlet of smaller turbo feeds into the compressor inlet of larger turbo, then the larger compressor outlet goes to the charge piping.

small pic, but decent example

could also be referred to as "Compound Turbocharging", but ive always known it as sequential

http://en.wikipedia.org/wiki/Twin-turbo