It is a characteristic of English (and the English accent in other languages) that unaccented neutral vowel sounds, especially before 'r' or 'l', tend to become a schwa. A schwa sound can therefore be represented in English by any vowel. In most dialects, for example, the schwa sound is found in the following words:
For non-English speakers, it may be useful to know that the sound is very similar to a short French unaccented e, or a German ö (an o with umlaut). It is a central, half-open vowel, exactly in the middle of the International phonetic alphabet vowel chart.
Quite a few languages have a schwa sound. It is almost always unstressed. Bulgarian is a language that does allow stressed schwas.
Some browser fonts will show the schwa symbol here: ə. Others may show either a box, a question mark, or capital Y.
The word "schwa" (shəwa, later shəva) originally referred to one of the vowel points used with the Hebrew alphabet, which looks like a vertical pair of dots under a letter. This sign has two uses, one to indicate the schwa vowel-sound and one to indicate the complete absence of a vowel. In practice these two uses do not conflict.