JAA is the European equivalent (an amalgamation of the individual national aviation authorities) of the FAA as far as licencing is concerned.
As for jobs, I suspect in the smallprint you may find they'll specify whether a JAA licence is necessary or whether an FAA licence is acceptable. If you were looking at a British company then I suspect a JAA licence would be required. HOWEVER, there are a lot of flying schools in the states that offer JAA licences, and I suspect would be able to advise on how easy conversion would be.
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