Schwa is the name linguists use for the most neutral of vowel sounds, the usual, weak, pronunciation of the English indefinite article a, the gentle grunts of uh huh. Its phonetic symbol is [ə] (which in Azeri is used as a letter).
In English, it can be represented by a number of vowel-letters: it is the u in careful, the e in worker, the a in above and the o in person.
Some languages have more than one schwa. In addition to the English one, Portuguese has [ɐ], which, as its 'a'-like symbol implies, is a little more open.
Russian unstressed o is pronounced schwa, identical to unstressed a, and, like a, it can occur in final position, a fact almost never reflected in English, which normally gives full value to final o in names like Yevtushenko and Chern(y)enko.
The name "schwa" is derived from a Hebrew word that means "emptiness" or "vanity", and it is also the name of a Hebrew vowel mark that is sometimes pronounced like a schwa, and sometimes not pronounced at all.