Muslims believe that all people are born with a natural faith in God. According to Islam, children are born with an innate sense of God, which is called the fitrah. Therefore, some people see conversion to Islam as a "return" back to this original, pure faith. For this reason, many Muslims prefer to say that they have "reverted," rather than converted to Islam. A common definition of the word "revert" is to "return to a former condition or belief."
Both terms are commonly used to describe those who embrace Islam as adults, after having been raised in or practicing a different faith system.