Australian slang term for a woman, or a man who is acting enough like one to justify being referred to as one. Pronounced the same as the name, which is also sometimes spelled 'Sheelagh'.
Sheila is mostly considered to be a slightly derogatory word, similar to the British use of the word 'bird' in certain circles. This is why it is definitely NOT the female equivalent of the word 'bloke' which actually has fairly positive connotations.
Sheila is also a term associated fairly strongly with bogans and country folk, and its use paints you in a certain 'lower class' light, or implies that you are trying to paint the woman in question in such a light.
If used to refer to a man, it is almost always prefaced with the word 'bloody', as in "Don't be such a bloody sheila mate". This is usually said in gentle jest, as there are a wide variety of alternative colloquialisms that would be far more appropriate if you really wanted to vent some spleen.
Be careful using this word. Some women will deck you for using it to describe them (and some Aussie women have a punch like an iron bar), and some men will do far worse.