Forex Trade

I cannot understand the ups and downs of foreign exchange. No two local banks have the same buying and selling rates. I try checking online the current forex rates and still no two sites have the same conversion, at least regarding the PHP to USD rates. What I still don't get is how forex sites earn money. I read that these sites have platforms wherein you can dabble in forex and ear money on the side. I wonder how it is done. Maybe I should look up on an offer I received about a Forex mentoring program. Who knows? I might get to learn a few things about forex and maybe invest a few bucks on the side. I'll have a check and let you know the results.