Aspiration

Aspiration is to [h] what prenasalization is to nasals, and frication to affricates: a merging of the sound as a part of a build-up or release of another, usually a stop. It is denoted in the IPA by a subscript <h>: [ʰ]. There are two main types:

Postaspiration

By far the most common type of aspiration is postaspiration of voiceless stops. Postaspirated voiceless stops (and affricates) are found phonemically in many languages, including eg. most of the Indo-Aryan, Caucasian, and Na-Dene families. They are found allophonic to plain voiceless stops in eg. Germanic languages. They are rather stable but may decay to fricatives (likely via a brief affricate stage, the remains of which still exist in e.g. German). A less usual outcome is buccalization of the aspiration, which produces clusters of stop/affricate + a dorsal fricativ /x/ or /χ/, as found in eg. Navajo.

Aspirated voiced stops are less common. Usually this involves breathy voice on the actual stop portion and/or during the release, which may be captured by the IPA symbol [ʱ]. They're reconstructed for Proto-Indo-European and survive in the Indo-Aryan branch (alongside the newer voiceless aspirate stops). Their fate in the other branches demonstrates their greiter instability: they became voiceless (post)aspirates in Greek and Italic, voiced spirants in Iranian and plain voiced stops in most other branches. (The last scenario may however be misleading, since these voiced aspirates most likely originate from pre-IE plain voiced stops, and so some branches may have never had them at all.) Voiced aspirates are passingly recorded in other language families, too, usually as an intermediate stage on a pathway from voiced stops to voiceless aspirated stops. Currently they can be found in eg. southern Bantu languages such as Xhosa.

Aspirated continuants are rare, but attested. Two examples are Burmese (with /sʰ/) and Sanskrit (with aspirated liquids).

Preaspiration

Preaspirated consonants are basically limited to voiceless stops in the postvocalic position. This is a rather unstable type, and will easily decay to a sequence of stop + homorganic fricativ (e.g. [ʰk] → [xk]) or plain voiceless stops (possibly with compensatory lengthening of a preceding vowel). Preaspiration may very rarely contrast with postaspiration, but a more common pattern is (medial) preaspiration being allophonic with (initial) postaspiration.

Preaspiration can rise from the lenition of voiceless fricativs (e.g. [sk] → [ʰk]) but also from gemination and possibly preglottalization.

Source

Silverman, Daniel (2002): On the rarity of preaspirated stops, Journal of Linguistics #39 / 2003.