^^ lol bro, he's got 10 posts...most likely he doesnt know his way around the forums yet or even the site..just help out w/out the comment
anyway, as already said its your cars computer. it's what checks everything and makes sure everything is running right. it does everything from checking to make sure you have enough oxygen in the air so your engine can run properly to making the seatbelt light flash when your not buckled up.
the only ecu replacment for celica is the power fc..you should have learned a lot about it if you clicked the link which cycocelica gave you..it's basically a retuning module and is VERY helpful if you've had aftermarket modifications done AND a dynonameter.