Purpose of text
What a text is trying to communicate to its target audience and the responses it seeks from the reader or listener. The purpose may be to create a voice that entertains, informs, expresses feelings, persuades etc, but remember you need to say more than just 'the purpose of this piece is to entertain.' E.g. ...to entertain through the use of...to persuade people to believe...and to buy...to express deep sadness by...etc
Audience of text
Who the text is written for or aimed at. The audience can vary from general (such as adults), to the very specific (female twenty-somethings who work in the city). The voice created by the language reflects the audience it is intended for. Note also that a text may have several different audiences at the same time. A speech in the House of Commons by the Prime Minister may have an immediate audience of the other MPs, but it may also be aimed at the wider audience watching on television and then perhaps also more specifically, undecided voters.
Context of text
The context is the situation in which a text is produced and recieved. These will both affect interpretation and how the meaning is created. The voice adopted will depend on the context. For example, the sentence 'I love you' could have very different meanings depending on the context in which it is said and heard. Justin Beiber saying it to a crowd of 100,000 fans is very different to it being said by a drunk teenager to a girl he has just met at a party. You might like to have a bit of fun exploring how many different meanings this sentence can have in different contexts. Another aspect of context is genre. A genre is a style or category of text (e.g. prose, poetry, drama). There are sub-genres within these broad categories (e.g. romance, historical novel, biography, dramatic monologue, epistolary novel, chick lit, short story etc. Each genre has characteristic literary and linguistic features.