Grey literature can be defined as materials and documents, usually intended for a specialised audience, produced by organisations outside of the traditional commercial or academic publishing and distribution channels. Harry Potter and the Philosopher's Stone is published - and publicised - by the commercial publisher Bloomsbury and is available in bookshops and libraries. The 2017/2018 annual report of Athlone Institute of Technology (AIT) in comparison, won't be available in bookshops or libraries, nor will you see any advertising for it. The Harry Potter books are very popular, AIT annual reports less so. The 2017/2018 annual report of AIT is therefore a good example of grey literature.
In addition to annual reports of AIT and other institutions, there many kinds of information sources that are classed as grey literature, including:
Standards are documents that establish specifications and procedures designed to ensure the reliability of materials, products, methods and services and therefore ensure compatibility between products from different suppliers. They are produced by national, international and professional organisations, such as
Grey literature is an important information source because its content is quite unique and service niche areas
The Internet has made it much easier to search for and retrieve grey literature, so it's now far more accessible than it ever has been in the past. Grey literature can be very current because it does not follow traditional publishing pathways. On the other hand, most grey literature doesn't go through the peer review process, so there may be issues regarding quality. On the other hand, not having having to undergo peer review means the grey literature is often more up-to-date than scholarly sources. Another thing to watch for is that some organisations e.g political "think tanks" may have their own agendas, producing material that is biased towards - or against - a particular point of view.
See the links page for websites to search for grey literature. You can also use Google or another search engine as a lot of grey literature is freely available on the web, though what you're looking for might be difficult to find, without trying the following
Note how all results - on the first page anyway - are PDF files.
Use the "site" filter to narrow your search to one particular website
Note how all results come from www.gov.ie
The examples above use Google, but other search engines will use similar, if not the same filters