Nigerian scam comes in many forms: 419 scam, when they offer to transfer millions of dollars into your bank account, or lottery scam, when they tell you that you've won something in some bogus lottery.
However, Nigerian dating scam (or romance scam), besides just asking for money for their studies, sick relatives, etc.. usually involves this scheme: the scammers upload fake attractive photos, in most cases of white people. They pretend to be the foreign specialists working in Nigeria or Ghana (usually originally from US and UK, but it may also be Canada, Australia or any other European country). Recently there are also Nigerian scams originating from Malaysia, China, India, Turkey, Spain, Cyprus, Egypt, as that's where Nigerians are located since they can easily get to these countries without visas or on the student visas.
Men are usually widowed engineers, building contractors, military, vet doctors, surgeons or antique dealers. Women are usually nurses, models, charity or UNICEF workers or antique dealers.