[Variously from Old English -ere, of Germanic origin; French -ier, from Latin -arius, -arium; Latin -aris; Old French -eure from Latin -atura; Old French -eor from Latin -atorium; Anglo-Norman French -eour or Old French -eor; Anglo-Norman French infinitive ending.]
This is a common and productive suffix, with several senses.
One large set comprises words for people who are concerned with a specified object, concept, or action, often as a trade or occupation, but frequently in some broader sense: abuser, banker, carpenter, designer, explorer, farmer, inspector, observer, philosopher, swimmer, writer. Many hyphenated forms exist: asset-stripper, house-hunter, record-breaker, theatre-goer, window-shopper.
A second group refers to a person who lives in a specified place: Londoner, Icelander, New Yorker, New Zealander. More broadly, it includes people who live in a certain type of place or who have an attribute related to place: city-dweller, foreigner, northerner, villager; insider and outsider are usually figurative rather than directly related to position.
Some informal, complimentary terms refer mainly to women: looker, stunner, goer, smasher (the sense overlaps with that discussed in -er3). Some, often of obscure formation, are abusive coarse slang: bleeder, bugger, fucker, nigger, tosser.
Some examples, chiefly in law, denote verbal action or a document effecting such action: disclaimer, waiver. These come from Anglo-Norman French infinitive endings, as do supper and dinner.
The ending also denotes objects that perform a specified action or activity, or have a given attribute: blotter, cutter, gasholder, lawnmower, liquidizer, roller, sweater. Again, many examples are hyphenated: back-hander, air-freshener, lemon-squeezer, quarter-pounder.
Some nouns in -er are accidental spellings, mostly from early English or Scandinavian sources: boulder, ladder, sewer (in the drainage sense), summer, tinder, tuber, winter; some US spellings are equivalent to British English -re: specter, theater.