ECMs and Chips

[Brodach]

I'm somewhat new to all of this, and I haven't had alot of time to read in and research alot of this, but what is the difference in getting your ECM reflashed and just buying a chip? I always assumed that the chip just plugged into the ECM and recalibrated it. I don't really know anything about the computers in cars, but I was wondering which yeilded better gains? Buying a nice chip, or just having your ECM reflashed... can you have both done? Prolly a stupid question, but I don't know alot about that field in cars. Thanks.