Can someone explain to me what the difference between a messianic jew and a Christian are ?

A Messianic Jew still thinks he has to observe the Law of Moses.

Jesus exists and is God.  God is Triune and Jesus is His Divine Son.

Just insert a demigod.

If they both believe the true Gospel, then there is no difference.  And everybody is supposed to be under grace.  Nobody is supposed to be under law.  Jewish people also not to be under law.  Jewish people also need to believe in the Lord Jesus Christ in order to avoid hell, like all people.  The Lord Jesus Christ is God and He loves you, and believing in the Lord Jesus Christ means being forgiven all sins past and future, and means going to heaven and not hell.  And all saved people still sin.  See 1 John 1:8.  Death leads to immediate heaven or hell, and it is too late to be saved, after death.  To be in heaven and not hell, believe in the Lord Jesus Christ, who is God and who died on the cross and shed His blood to pay for all of our sins in full, and who was buried, and who resurrected from the dead.  The only way to avoid hell is by believing in the Lord Jesus Christ, without adding any of your own works.  See Romans 4:5, 1 Corinthians 15:3-4, and John 3:16.

That's easy. ZERO. Messianic Jews are actually the 'original' Christians who first received the promised Holy Spirit on Pentecost, (Acts 2). Gentile Christians didn't receive the Holy Spirit until a later outpouring, (Acts 10). Jesus speaks of both of His 'folds' making up the One Flock over which He Is the Good Shepherd, with the second 'fold,' ('Gentiles'), coming in after the first, ('Jews'), and joining together into the One Flock, (John 10:16). 

That is exactly what happened. Both 'folds' have been in simultaneous existence since the Gentile outpouring of the Holy Spirit. This just isn't well recognized because non-Messianic Jews have a very strong public voice and don't like people knowing about them or their influence.

Messianic Jews combines the lifestyle and heritage of Judaism with the New Covenant teachings of Jesus as the only provision for salvation from sin and God’s New Covenant.  Both are Christian but non-Jewish Christians generally don't live the lifestyle and heritage of Judaism.

There’s no such thing as a “Messianic” Jew!   That’s a total misnomer. And when they say “messianic”, they mean Jesus. Jews wait for the Messiah, because we don’t believe it’s Jesus. If a Jew believes that Jesus is the Messiah, then he/she is an apostate and cannot take part in any rites or rituals of Judaism. 

Messianic Judaism is a modern syncretic religious movement that combines Christianity with elements of Judaism and Jewish tradition. It emerged in the 1960s and 1970s. Many Messianic Jews believe that Jesus is God the Son, and that the Tanakh and New Testament are the authoritative scriptures.  

According to clear scripture, the only true Jews identify Christ as Messiah. He is the author and finisher of the Hebrew faith, and Christianity is Judaism concluded in perfection.

Christ was a Jew in the flesh, Mary and Joseph, the apostles and the earliest church.

Hebrews 12:2 — We look unto Jesus the author and finisher of our faith; who for the joy that was set before him endured the cross, despising the shame, and is set down at the right hand of the throne of God.

Romans 2:28-29 — For you are not a true Jew just because you were born of Jewish parents or because you have gone through the ceremony of circumcision. No, a true Jew is one whose heart is right with God. And true circumcision is not merely obeying the letter of the law; rather, it is a change of heart produced by God’s Spirit. And a person with a changed heart seeks praise from God, not from people.

A Messianic Jew is a Christian Jew, a Jew who believes Jesus Christ is the Messiah and simultaneously adheres to Jewish religious traditions and practices, the two not being mutually exclusive. They're obviously not mutually exclusive, being that Jesus Christ himself was a Jew and even a rabbi, as were his apostles Jews and most of his disciples during his ministry.