Christian means you follow the creeds of the christian religion in which the "Jesus Christ" that Paul spoke of is the central figure and usually also divine. Often, no or few tenets of Judaism, commandments, feast days or torah laws of the Hebrew Bible (Torah and Tanakh aka Old Testament) are followed, as the Old Testament is viewed as nothing more than a historical document full of stories that are true historically but the instruction is no longer relevant.
Messianic Judaism takes many forms, but, in the simplest way possible, it can be either Jews who also believe in Yeshua (the true name of Jesus) as messiah, or gentiles who understand that Yeshua was the Jewish messiah and that the importance of the Torah laws and feasts never became irrelevant.
Subsects of MJism are usually separated by different levels of acceptance of Christianity's views (trinity, divinity of Yeshua, christian feasts and holidays, etc), an acceptance of Paul through a pro-torah filter (Paul-gnostic Messianic Judaism is the most common, Paul-rejecting Messianic Judaism is less common), an acceptance or rejection of Rabbinic or Talmudic Judaism (IE accepting the Talmud, the Kippah, the harsher/stricter sabbatical laws that are found in the Talmud, etc), whether or not they try to apply conspiracy theory related, Hebrew language or sacred name related Gnosticism to the faith etc...
Not only are Christianity and Messianic Judaism totally different, but the subsects are as different to eachother as, say, Mormonism is different to Catholicism, is different to Jehovah's Witnesses, is different to Baptists etc etc.
By applying such labels, I myself would be labelled as a Paul-rejecting Messianic Jew who ONLY follows Torah, the Tanakh (collectively, the old testament) and the Gospel of Matthew and the letter of James.I also believe that there is truth to some of the books found in the Dead Sea Scrolls, including the Book of Enoch.
I do not consider Yeshua a divine being, a sacrifice for sin/a replacement for Torah obedience and repentance of transgression, I do not accept the Pauline/Christian understanding of the "Holy Spirit" as a being or avatar of the Creator, but, as it literally says in Hebrew - the Ruach HaKodesh, the Breath (of life/of being set apart/of being sanctified/of our Creator) which may at times refer to the soul or non-physical essence of man, represented y our literal life, and at times refer to direct inspiration from our Creator, as are the clear meanings of Holy Spirit in the Tanakh.
That said, I do not really consider myself that, I don't like such labeling, I prefer to consider myself an Israelite.
I hope this helps (and that no one responds in any way so as to begin a debate or argument lol - though I am happy to answer any questions that only require a simple or short response as I am currently very sleep deprived).