A glottal stop is what happens when you contrict your vocal chords to shortly stop airflow completely. It often spaces sounds, particularly vowels.
Since it restricts airflow, it can be classified as a consonant, but (like vocal clicks) can also be considered separate from 'regular' vowel/consonant division.
Glottal stops are a more regular features of some languages, including Arabic, some Northern Canadian dialects.
They happen in more languages than you might think, often as an implication of pronunication more than than as an intention. For example:
- English "uh-oh" involves a stop between the uh and the oh, separating the two vowelish sounds.
Many people will also start the 'uh' with a closed glottis.
- Dutch's be·amen, for a similar reason.
- In the Cockney accent it often replaces the t at the end of a word, such as habit, pronounced more like habi'.
An apostrophe or (mid)dot are common ways to write such a stop, this this less part of everyday writing systems, and more part of phonetic transcription, and contructed languages (primarily fantasy and science fiction) like those in or science fiction. In the case of Star Trek, in particular Klingon, this came from the writers consulting linguist to design a believable and functional language.