Actual possession is defined as possessing the drug or contraband on your person. If the drugs/contraband is found on you, then the question turns to whether or not the office had reasonable suspicion to conduct the search that led to your arrest.
The concept of Constructive Possession is much more complicated. Constructive possession occurs when the drug/contraband is not found on a person. Rather, the drug/contraband in question is found in another location or common area.
CONSTRUCTIVE POSSESSION: GOVERNMENT'S BURDEN OF PROOF
When charged with constructive possession, the government must prove 3 things beyond and to the exclusion of every reasonable doubt: 1. The drug/contraband was within the person's dominion and control; 2. The person new of the existence of the drug/contraband; 3. The person knew of the illicit nature of the drug/contraband. Keep in mind that the law says more then 1 person can be in possession of the drug/contraband at one time.
SWORN MOTION TO DISMISS
Constructive possession cases are often subject to Sworn Motions to Dismiss because they lack proof as to element number 1. The issue of dominion and control requires more evidence then the drug/contraband simply being within a person's reach. Items #2 & 3 are not subject to such motions to dismiss because the issue of knowledge is always left to a jury to decide.