Spirants involve the same restriction of the speech canal as fricatives, but the speech organs are substantially less tense during the articulation of a spirant. Rather than friction, a resonant sound is produced at the place of articulation.
Basically, friction and fricatives develop from tense articulations; when the articulation is lax, resonance, and thus a spirant, occurs. Also realize that many spirants can be thought of as the lax counterparts of stop consonants. These correspondances are noted in the descriptions below.
You can hear each sound by clicking on his IPA transcription.
Voiceless bilabial spirant. The IPA uses the same symbol for the spirant and the fricative. If there is a great deal of muscular tension, the sound is a fricative; if not, the result is a spirant. For more details, see the general description of fricatives. This spirant can be considered the lax realization of the stop [p].
Voiced bilabial spirant. Same as above, but with vibration of the vocal cords. This symbol represents the fricative as well as the spirant, according to the degree of tension of the articulators. The spirant is the lax counterpart of the stop [b].
Voiceless dental spirant. The tongue tip is held close to the upper teeth, either behind them (dental) or just underneath them (the interdental articulation). This spirant is the lax counterpart of the the stop [t]. Considering its place of articulation, it is unimportant to class this sound as dorsal or lateral.
Voiced dental spirant. Same as above, but with vibration of the vocal cords. This spirant is the lax counterpart of the stop [d].
Alveolar spirant. The tongue tip, without much muscular tension, approaches the alveolar ridge. No groove is formed, as opposed to the case of the hissers and hushers. The same symbol is used regardless of voice; the voiced variant is the typical American 'r'.