ECU flash tune VS powercommander /dyno tune diference

[storm]

i am trying to get an understanding of this whole tuninmg process so my limited understanding is
1) the ECU is COMPUTER THAT manages the engine running parameters. it has its 'software' from suzuki

2) flash tune- using a set up by woolich or other tuners the 'suzuki software ' on the ECU is reprogrammed.
this is a windows operation that is displayed on your computer screen and allows the user/tuner to change parmeters such as AF ratio,RPM, and other parameters without the bike on a dyno.
there are also maps or programs available if the user wants a generic program

3) the powercommander or anyother similar device is a 'piggyback' computer that allows the user to change the parameters of the engine management system without 'rewriting the original suzuki software'.
a dyno is needed( maybe not) to 'load the bike' and change the software as needed. there are also generic programs available or programs can be custom written

is my inference of the situation correct? can the gurus shed some light on this subject please?
what are the advantages/ disadvantages of either system types? thanks