First, there is complete closure. Plosives (= stops) have a stricture of complete closure somewhere in the oral cavity; additionally the velum is raised to shut of the nasal passage. During this stage air is compressed behind the place of closure and when the articulators part quickly, a short puff of air is audible. There are sounds other than plosives which also require a complete closure (e.g. taps) about which we will talk later.
The second degree of stricture may be termed close approximation. The articulators are close to each other but there is a narrow opening through which the air has to force its way resulting in turbulent airflow - something we perceive as friction. Fricatives are formed like this.
The third degree of stricture is characterised by open approximation. The vocal tract is wide enough so that there is no audible friction. Some academics call them approximants, others sonorants, still others resonants. Sounds produced with open approximation will be dealt with in a separate blog entry, which will also dwell upon some additional aspects of segmental articulation.