User Defined Function and regular Function

I read documentation, but have some questions: 1) What is the real difference between UDF and just creating function in sql? I mean the speed of execution. 2) If I create a function, is it implicitly C? I am asking because there is python binding also. 3) If I have already installed Monetdb and want to create UDF, should I recompile whole monetdb? I am not good enough yet to do something like this. I am asking, because I want to take a text type column full of 0s and 1s ("110101010"), represent it as a "int" or "bit" and do bitwise AND against other columns. When I am saying represent as int/bit, I mean not to change ("110101010") into bit, just let C know that it should interpret that number already as int or bit.