The Uniform Code of Military Justice differentiates between desertion and AWOL as follows:

Article 85 (Desertion)
(1) without authority goes or remains absent from his unit, organization, or place of duty with intent to remain away therefrom permanently;
(2) quits his unit, organization, or place of duty with intent to avoid hazardous duty or to shirk important service; or
(3) without being regularly separated from one of the armed forces enlists or accepts an appointment in the same or another one of the armed forces without fully disclosing the fact that he has not been regularly separated, or enters any foreign armed service except when authorized by the United States; is guilty of desertion.

Article 86 (Absence Without Leave)
Any member of the armed forces who, without authority--
(1) fails to go to his appointed place of duty at the time prescribed;
(2) goes from that place; or
(3) absents himself or remains absent from his unit, organization, or place of duty at which he is required to be at the time prescribed; shall be punished as a court-martial may direct.

However, Article 2.a(3) excludes Bush, since it says that the UCMJ only covers Members of a reserve component while on inactive-duty training, but in the case of members of the Army National Guard of the United States or the Air National Guard of the United States only when in Federal Service. "When in Federal Service" means when an individual or guard unit have been activated. If anyone is really bored, the entire law is here.