Deposition is the removal of a man from clerical office and return to the rank of layman. When a clergyman is deposed, he is no longer a clergyman and may not function in that role. Deposition is performed only by the episcopacy, and it is often the canonical penalty for committing acts of immorality or disobedience.
Clergy may be deposed by means of a formal, canonical process via a spiritual court, but they may also be deposed by more "existential" means, e.g., by leaving the Church. In either case, any action on the part of the episcopacy is really just a recognition of the self-deposition that a man has taken upon himself by means of his actions.
Orthodox theology of ordination does not teach that an "indelible mark" is conferred as in Roman Catholicism. Rather, because the function of the clergy is only within the context of the Church community and as an extension of the authority of the bishop (or in the case of a bishop, of the whole episcopacy), when a cleric is deposed, he is really no longer a clergyman.
Deposition is distinct from laicization in that the latter is usually the result of a request by the cleric to be returned to the ranks of the laity. The terms are sometimes used interchangeably, however. In addition, defrocking is sometimes used to refer to deposition, but this is less correct, as it reflects the Roman Catholic "indelible mark" theology (and thus, since a man is always a clergyman, he may only be "defrocked," i.e., no longer permitted to serve as a priest).
Being deposed is also distinct from suspension in that it is a permanent revocation of the clerical office, while suspension means only that a clergyman cannot serve for a temporary period.