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The vowels 
indicate a preceding palatal consonant and with the exception of 
are iotated (pronounced with a preceding /j/) when written at the beginning of a word or following another vowel (initial was iotated until the nineteenth century). The IPA vowels shown are a guideline only and sometimes are realized as different sounds, particularly when unstressed. However, 
is used in words of foreign origin without palatalization and indicate /e/. Which words this applies to must be learned (generally to avoid using 
after a consonant), and 
between soft consonants, such as in 
("toy ball").
is an old Common Slavonic tense intermediate vowel, thought to have been preserved better in modern Russian than in other Slavic languages. It was originally nasalized in certain positions: 
("rock"). Its written form developed as follows:
was introduced in 1708 to distinguish the non-iotated/non-palatalizing /e/ from the iotated/palatalizing one. The original usage had been 
had dropped out of use by the sixteenth century. In native Russian words, is found only at the beginnings of words, but otherwise it may be found elsewhere, such as when spelling out English or other foreign names, or in words of foreign origin such as the brand-name Aeroflot 
introduced by Karamzin in 1797 and made official in 1943 by the Soviet Ministry of Education,[4] marks a /jo/ sound that has historically developed from /je/ under stress, a process that continues today. The letter is optional (in writing, not in pronunciation): it is formally correct to write 
for both /je/ and /jo/. None of the several attempts in the twentieth century to mandate the use of have stuck.
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http://russian-alphabet.com/vowels.htm