What are the differences (main ones) between Orthodox, Reform, Conservative, and Messianics?

Your question makes me think of the old Sesame Street show song, "One of these things is not like the others, one of these things just doesn't belong. Can you say which one is not like the others, before I finish my song?" And just as the answer was always obvious on the show, it is here as well. 

 

Simply put, Orthodox, Reform, Conservative, (and Reconstructionist, Renewal, and multiple other groups) are Jewish, accepted within the bounds of Judaism, and include only those who do not worship Jesus and are not Christians. 

 

Messianics ("Jews for Jesus," so-called "Completed Jews", and others with various names) are simply not Jewish, are not accepted as Jews and never will be. They are Christians who worship Jesus.  

 

The two faiths are completely incompatible; one cannot hold both and be faithful to either. 

Anyone claiming otherwise is either completely deluded themself, or attempting to deceive others.  

 

{Administrator's Note: Since this is not a matter of conjecture or opinion, but is universally accepted in the Jewish world, and this is not an ethical, moral, values, or proper behavior subject, this is the only response this question (or any like it) will receive. No further questions regarding this topic will appear on the Jewish Values Online website.]