First, lets look to the statutes:
§ 18.2-18 - In the case of every felony, every principal in the second degree and every accessory before the fact may be indicted, tried, convicted and punished in all respects as if a principal in the first degree.As you can see, Virginia does not statutorily define any of these various ways of participating in a crime. While we can infer there are two types of principals (1st and 2d degree), two type of accessories (before and after the fact), and one type of conspirator, the statutes provide little guidance outside of punishment and charging. So, as in much of Virginia law, we have to look to the cases for actual definitions.
§ 18.2-19 - In the case of every felony, every accessory after the fact shall be guilty of a Class 1 misdemeanor; provided, however, no person in the relation of husband or wife, parent or grandparent, child or grandchild, brother or sister, by consanguinity or affinity, or servant to the offender, who, after the commission of a felony, shall aid or assist a principal felon or accessory before the fact to avoid or escape from prosecution or punishment, shall be deemed an accessory after the fact.
§ 18.2-21 - An accessory, either before or after the fact, may, whether the principal felon be convicted or not, or be amenable to justice or not, be indicted, tried, convicted and punished in the county or corporation in which he became accessory, or in which the principal felon might be indicted. Any such accessory before the fact may be indicted either with such principal or separately.
§ 18.2-22 - If any person shall conspire, confederate or combine with another, either within or without this Commonwealth, to commit a felony within this Commonwealth, or if he shall so conspire, confederate or combine with another within this Commonwealth to commit a felony either within or without this Commonwealth, he shall be guilty of a felony which shall be punishable as follows . . . [various grades of felony punishments].
A principal in the first degree is an active mover in the crime. Johnson v. Commonwealth, 220 Va. 146 (1977). He's the one who shoplifts steak from the Mega-Mart or runs into the bank carrying a shotgun.
A principal in the second degree is someone who procures, encourages, countenances, or approves the criminal act. McMorris v. Commonwealth, 276 Va. 500 (2008). He is guilty as a principal in the second degree if he is guilty of some overt act done knowingly in furtherance of the commission of the crime, or if he shared in the criminal intent of the principal committing the crime. Id. This is the look out man or getaway driver.
An accessory before the fact is the same as a principal in the second degree except that the person is not present when the crime is actually performed. Sutton v. Commonwealth, 228 Va. 654 (1985). This is the guy who provides plans to the bank, but is in Idaho on the day the robbery takes place.
Under the common law an indictment was required to plead specifically if a defendant were to be convicted as a principal in the second degree or as an accessory before the fact. However, § 18.2-18 changed that so that an indictment which alleges participation as a principal in the first degree is sufficient if the defendant instead turns out to be a principal in the second degree or an accessory before the fact. Id.
Accessory after the fact has three elements: (1) The felony must be completed. (2) The accused, at the time he assists or comforts the felon, must have notice, direct or implied, that the felon committed the crime. (3) Third, the accused must receive, relieve, comfort, or assist the felon. Commonwealth v. Dalton, 259 Va. 249 (2000). Of course, § 18.2-19 exempts close family members and servants from punishment if they violate this law.
A conspirator is one of at least two people who agree to commit a crime. The crime does not have to succeed and there does not even need to be an act in furtherance of the conspiracy. When the agreement is reached the conspiracy is committed. Although each conspirator must agree on the same objective, a conspirator is not required to know all the details of the conspiracy, the identity of the other conspirators, the part each member of the conspiracy is to play, or how the spoils of the conspiracy are to be divided. James v. Commonwealth, 53 VaApp 671 (2009).
In most cases, a conspirator is also a principal or accessory. However, because conspiracy requires a meeting of the minds, but no actual act in furtherance, one could be a conspirator without getting far enough along to become a principal or accessory. For instance, Mr. Pink and Mr. White meet randomly at Subway one day. Each recognizes the other from a previous robbery they committed. They sit to eat lunch together and while eating agree to rob the bank next door. However, before they can even leave their table, the off duty deputy who was eating lunch at the next table and heard them agree to rob the bank arrests them. Mr. Pink and Mr. White are clearly conspirators, but it is hard to argue they are principals or accessories because they were stopped before they could undertake any actual act.