Is it true that egyptian arabic is the most common arabic language?
It's a dialect not a language.
I think most of the Arabian countries watch the Egyptian movie, plays and things like that, that's why it's well known between the dialects, however each country speaks it's own dialect.
is it true that there is difference between arabic and modern standard arabic?
I think you mean the difference between Classic Arabic and Modern Standard Arabic.
Yes, there's a difference, the CA used to read the Holy Qura'an.
MSA: used in TV shows, scientific and educational programs, books and novels, and things like that.
Non of them is spoken in Arab countries, because as I mentioned we speak the DIALECT.