As the amount of information we are subjected to each day increases, it's critical that we strive for clear, succinct communication. However, two tools at our disposal that often negate their use are acronyms and abbreviations. They are intended to help the writer communicate their ideas more efficiently without needing to be overly verbose. Used properly, they can both be a time saver for the author and reader. However, careless usage ultimately results in the author not being understood.
Let's start with the use of acronyms. In written form, it's acceptable to introduce a small number of acronyms to avoid the reader needing to digest a long phrase or word frequently repeated within a text. However, if too many acronyms are introduced, the text can start to resemble alphabet soup and be difficult to read. In a document, it's good practice to include a glossary to help the reader quickly look up what a an acronym is. I personally take the approach of reintroducing the acronym in each chapter to remind the reader in order to reduce the number of times they need to flip to the front of the document to remind themselves.
Presentations are frequently a victim of the overuse of acronyms, often without explanation. During a presentation, there's no glossary and you can't readily lookup the acronym - though you should ask the presenter for clarification. It's often the case that acronyms are used in presentations to maximise the use of available space. Given the purpose if effectively communication, cutting corners can have significant negative impact. Either the audience doesn't ask for clarification and you lose them or they get distracted trying to guess what you are talking about and ... you've lost them. My advise is, keep your use of acronyms to a minimum and always spell them out for the audience if there is any doubt of their meaning.