A SID (Standard Instrument Departure) is a procedure for departing an airport and joining the airway structure. A STAR (Standard Terminal Arrival Route) is a procedure from the airway structure onto an approach.
Flights under Instrument Flight Rules (ifr-navigation) are usually assigned a Standard Instrument Departure (SID) when departing an airport and a Standard Terminal Arrival Route (STAR) when arriving at an airport. Not all airports use SIDs or STARs though.
Both procedures consist of a set of waypoints, optionally with altitude and/or speed restrictions, to organize the traffic flow in the area around the airport. A SID begins at the departure runway and ends at a waypoint where one or more airways can be joined. A STAR begins at such a waypoint and leads to an Initial Approach Fix (IAF), where an approach begins.
Use this tag for questions about the departure and arrival procedures. When specifically asking about departures, use departure-sid instead.