An abbreviation is a shortened form of a word, or phrase, typically created by taking initial letters for example OEM for Original Equipment Manufacturer or shortening the word, for example 'Tues' for Tuesday.
An abbreviation can be an acronym (e.g. AIDS), initialism (e.g. OEM), or neither (e.g. Tues.). The latter is sometimes referred to as a truncation, but it is more often called merely an abbreviation.
Acronym-based abbreviations are generally pronounced as a word, for example 'radar' (/ˈreɪdɑː/) and 'AIDS' (/eɪdz/).
Initialism-based abbreviations generally ignore prepositions and determiners, for example 'FBI' is an abbreviation of 'Federal Bureau of Investigation'.
Truncation-based abbreviations are commonly denoted by a full stop or period, for example 'Messrs.', but this is often ignored for the most common abbreviations such as 'Mr'.