What is the difference between I'll and I will?
what is the difference between apostrophe words and non-apostrophe words?
When can I use I'll?
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The difference between "I will" and "I'll" is their formalities. Contractions are generally less formal than the same two words written out.
You can use contractions in informal speech and messages, but not in formal, professional writing. Many style guides for journalism and academic writing discourage the use of contractions outside of quotes.
Further reading: http://www.everythingenglishblog.com/?p=558