To search for words on all pages, just enter the words in the search box at the top and click "search" (or press enter). You should see a list of pages that contain all of the words you entered. Note, that some words, so called "stop words", are so common, that searching for them doesn't make sense – these words will be removed from your query, unless they are the only words you are searching for. The list is sorted by the relevance of the words, and the number of times they appear.
If you click on "search" without entering any words, you will see an index of all pages in the wiki.
By clicking the "Backlinks" link at the bottom of every page, you can search for all pages that link to it. This is useful for categorizing pages – just put a link to the category page on every page from the category.
Images and other binaries (pdf or doc documents, for example) are not indexed and therefore you can't search in them – you can only search for their name.