The term colleague comes from the Latin ‘collega’, meaning ‘partner in office’, and it is normally used to refer to someone who works with you in the same organisation. Being your peer, the individual is usually of the same rank or level as you, and the two of you have more or less the same set of responsibilities. A teacher, for example, usually refers to her fellow teachers as colleagues.
The word co-worker suggests that the other person works in the same organisation as you; the two of you, however, may not be working together, and as a result, may not be sharing the same set of responsibilities – you may be a teacher and your co-worker may be the personal assistant to the Vice Chancellor. Unlike a colleague, a co-worker may be of a higher or lower rank than you. These subtle distinctions, however, are not always maintained in everyday conversation. In India, we tend to use colleague to refer to anyone who works in the same organisation as we do.
By the way, the vowel in the first syllable of colleague sounds like the ‘o’ in ‘got’, ‘cot’ and ‘not’, while the second syllable is pronounced like the word ‘league’. The word is pronounced ‘KO-liig’ with the stress on the first syllable – many of us in India tend to put the stress on the second.
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