In British usage, some words of French, Latin, or Greek origin end with a consonant followed by -re, the -re is actually pronounced /ər/. In the USA most of these words (note "most" not all) have the more phonetic spelling of -er. The difference is most common for words ending -bre or -tre: British spellings centre, kilometre, litre, lustre, mitre, nitre, reconnoitre, saltpetre, spectre, theatre, titre, calibre, fibre, sabre, and sombre all have -er in American spelling.
Surprise surprise, there are many exceptions to the -re spelling in British usage. Many words spelled with -re in Modern French are spelled with -er in both British and American usage; among these are chapter, December, diameter, perimeter, disaster, enter, filter, letter, member, minister, monster, October, November, number, oyster, parameter, powder, proper, September, sober, and tender.
The ending -cre, as in acre, lucre, massacre, mediocre, is preserved in American English, to indicate the c is pronounced /k/ rather than /s/.
After other consonants, there are not many -re endings even in British English: louvre and manoeuvre after -v; meagre (but not eager) and ogre after -g; and euchre, ochre, and sepulchre after -ch. In the United States, ogre and euchre are standard; manoeuvre and sepulchre are usually spelled as maneuver and sepulcher; and the other -re forms listed are less used variants of the equivalent -er form.
Even American-derived forms of nouns and verbs, for example, fibers, reconnoitered, centering, are, naturally, fibres, reconnoitred and centring respectively in BrE. Centring is a particularly interesting example, since it is still pronounced as three syllables in British English (/ˈsɛntərɪŋ/), yet there is no vowel letter in the spelling corresponding to the second syllable. It is dropped for other derivations, for example, central, fibrous, spectral. However such dropping cannot be regarded as proof of an -re British spelling: for example, entry derives from enter, which has not been spelled entre for centuries.
The difference relates only to root words; -er rather than -re is universal as a suffix for agentive (reader, winner, user) and comparative (louder, nicer) forms. One consequence is the British distinction of meter for a measuring instrument from metre for the unit of length. However, while poetic metre is often -re, pentameter, hexameter, etc. are almost always -er.
Many other words have -er in British English. These include Germanic words like anger, mother, timber, water and Romance words like danger, quarter, river.
AmE has exceptions too: Theater is the prevailing American spelling used to refer to both the dramatic arts and buildings where stage performances and screenings of movies take place (i.e., "movie theaters"); for example, a national newspaper such as The New York Times uses theater throughout its "Theater", "Movies", and "Arts & Leisure" sections. In contrast, the spelling theatre appears in the names of many New York City theaters on Broadway (cf. Broadway theatre) and elsewhere in the United States. In 2003 the proposal of the American National Theatre, eventually to be founded and inaugurated in the fall of 2007, was referred to by the New York Times as the "American National Theater"; but the organization actually uses "re" in the spelling of its name. The John F. Kennedy Center for the Performing Arts, in Washington, D.C., or The Kennedy Center, features the more common American spelling theater in its references to The Eisenhower Theater, part of The Kennedy Center. Some cinemas outside New York use the "theatre" spelling.
In many instances, places in the United States use Centre in their names. Examples include the Stonebriar Centre mall in Frisco, Texas, the cities of Rockville Centre, New York and Centreville, Illinois, and Centre College in Kentucky. Sometimes these places were named before spelling changes took effect, but more often the spelling merely serves as an affectation. There are also a few cases of the use of Center in the United Kingdom (e.g., the Valley Centertainment in Sheffield, although this is in fact a portmanteau of "centre" and "entertainment").
For British accoutre(ment), the American practice varies: the Merriam-Webster Dictionary prefers the -re spelling, but the American Heritage Dictionary the -er spelling.
More recent French loanwords retain an -re spelling in American English. These are not exceptions when a French-style pronunciation is used (/rə/ not /ər/), as with double-entendre, genre, or oeuvre. However, the unstressed /ər/ pronunciation of an -er ending is used more or less frequently with some words, including cadre, macabre, maître d', Notre Dame, piastre, and timbre.
Commonwealth usage.The -re endings are mostly standard throughout the Commonwealth. The -er spellings are recognised as minor variants in Canada, due in part to American influence. Proper names, particularly names incorporating the word centre/center, are an occasional source of exceptions, such as, for example, Toronto's controversially-named Centerpoint Mall.