Source: http://www.interjobs.co.uk/searching/hidden.html The wider your network, and the more often you use it, the better it works. At a low and broad level, friends, relatives or people you meet may know where there are jobs that are not advertised. Always ask them, as their employer may be looking for staff. This search method cannot be undervalued.
Source: http://www.interjobs.co.uk/searching/hidden.html On another level, your network may include fellow professionals, past employers or similar people. Networks need to be maintained regularly (even when you're in employment) to get them to work effectively for you.