A turn is the time during which a single player has priority and takes his/her actions. In a game of Android: Netrunner, the Corporation player and Runner player alternate taking turns, and each sides' turn has different rules. Both the Corporation and the Runner have an Action Phase and a Discard Phase. The Corporation is the only player with a Draw Phase. Once a player has gone through all the phases of his/her turn, play passes to the other player and that player takes his/her turn.
The first part of the Corporation's turn is the Draw Phase, wherein s/he draws the top card of R&D. The rest of the turn proceeds as normal, with an Action Phase and a Discard Phase. During the Corporation's turn, that player has to spend performing actions. After the Discard Phase, the Corporation's turn ends, and the Runner's turn begins.
The Runner does not have a Draw Phase, but instead only has the standard Action and Discard Phases. During the Runner's turn, that player has to spend performing actions. After the Discard Phase, the Runner's turn ends, and the Corporation's turn begins.