Acad. V.V. Vinogradov established the following classification of synonyms: ideographic, stylistic, ideographic-stylistic, contextual, absolute (total).
Ideographic synonyms are words conveying the same notion but differing in shades of meaning or emotions expressed: a piece - a lump - a slice; to tremble - to shiver - to shudder; anger - fury; to like - to admire - to love.
Stylistic synonyms are words conveying the same notion but different in stylistic characteristics:
to begin - to start - to commence;
sky - heaven (poetic);
to see - to behold (archaic);
horse - steed (poetic);
to try - to endeavour (bookish).
Stylistic colouring may also be accompanied by a difference in emotional colouring or some other shades of meaning:
to say - to pronounce;
head - onion;
money - cabbage;
face - puss.
Such synonyms are called ideographic-stylistic.
Contextual synonyms are similar in meaning only under some specific distributional conditions. Thus, the words bear, stand, suffer are synonyms only when used in the negative form: can't stand it - can't suffer it - can't bear it. Otherwise, all these verbs are semantically different.
Absolute (total) synonyms are words coinciding in all their shades of meaning and in all their stylistic characteristics. Absolute synonyms are usually technical and scientific terms, they are rare in the vocabulary and temporary. The vocabulary system tends to reject one of the absolute synonyms or to develop differentiation of characteristics in one or both (or all) of them: noun -substantive; flection - ending; oculist - eye-specialist.