How To Use The Search Tool
This article should hopefully provide general guidance to those who don't know how to use the search function. I've also gone into detail about search operators and advanced functions, which may give regular users some hints and tips for increasing the effectiveness of your searches.

What is the search tool? Why use it?
The search tool provides an interface to pick out particular information from all the posts in the forum, thus saving having to repost questions that have been asked before or assist in finding old threads. It looks through every available post in the forum database and returns with either threads or specific posts (depending on search terms) that match your search string.

Why do you mention a Search Tool and a Search Engine?
Just to define two of the terms I use in this guide:

Search Tool is the little box you type your text into. Its the form that takes your search string (the text you search for) and options you select in the Advanced Search form, then passes it on to the Search Engine when you click "search" or "go". Basically, its the front end that you interact with.
Search Engine is the code of the forum that performs the physical search of the database. It takes input from the Search Tool (i.e. search string and options) and uses that to interogate the database to bring up results, which are then displayed to you. Basically, its the code that physically performs the forum search.

How to access the Search Tool.
To access the Search Tool, click the Search link (blue text, under the header that lists your location within the forum). The quick search box will appear, you can either type your search string in here or click the Advanced Search link for more options (see below for descriptions of the additional options).

What is a "search string"?
Whatever you input into the search box (in quick search, or advanced search) is your "search string", of one or more words. For improved results, keep your search string short and specific. By default, the search engine looks for threads that contain ALL words in the search string (not necessarily in the same post).

Minimum/maxiumum length words
To keep search results within a reasonable count and to obtain best accuracy, each word in the search string has to be 5 characters or more. So searching for "20XE" will fail, where as "C20XE" will return a list of results. The maximum length of a searchable word is 20 characters.

Quotation marks
Unfortunately, the search engine in vBulletin 3.5.3 is incapable of handling quotation marks to search for a specific phrase of words, so there is no requirement to quote the search string (or parts of it). The search engine ignores quotation marks - searching for C20XE engine conversion is the same as searching for "C20XE engine conversion".

Search Operators
Some special characters can be used by the search engine. By default, every word in the search string is looked for, however certain operators can affect how the search engine handles words.

Operator: +word
Function: Include this word in the search

Operator: -word
Function: Exclude results that include this word

Operator: word1 OR word2
Function: Search for either word1 or word2

Operator: <word
Function: Reduce relevance of word (for use with relevance ordered results, see below)

Operator: >word
Function: Increase relevance of word

The wildcard, or asterisk, character allows character replacement. The asterisk allows searching for one or more character - so searching for conver* will bring up converse, conversion, conversation etc. It can also be inserted into a word - so con*ion will return conversion, conversation, etc.

Advanced Search
The advanced search form has more options for searching and displaying results, as follows.

Key Word(s)
This is the search box, enter your search string here. Options are to search entire posts, or just the titles - thats pretty self explainatory. Bear in mind, when used with other options on the form, the Key Words box is not a required field - leave it blank if you just want your search results to be based on selections made in other parts of the search form.

Search By User Name
Here you can search for posts or threads by a specific user. This will apply the search string in the key words box to that particular user and only search within their posts, or their threads (depending on Find Posts By User/Find Threads Started By User selection). If the Key Words box is blank, the search engine will pull up all results (posts or threads) that apply to the user listed here.

Note that this is the same result as going to a user's profile and clicking "show posts by user" or "show threads by user".

Find Threads With
This will filter results to the number of replies. Options are either More Than or Less than, and action on the number input into the text box there. Default is "at least 0 relpies", so all threads are searched.

Find Posts From
To search within posts that are newer or older than a given time span, use this option. Default is any date, so all posts in the forum database are searched.

Sort Results By
To change how the list of search results are displayed, change the options here. Options are:

Option: Last posting date
Result: Results with the newest replies go at the top of the list (default)

Option: Relevancy
Result: Most relevant results are displayed at the top of the list (for use with < and > operators)

Result: Sort results alphabetically by thread/post title

Option: Number Of Replies
Result: Results with the most replies come first

Option: Number Of Views
Result: Results with the most views come firs

Option: Thread Start Date
Result: Newest threads come first on the list. For posts, this is calculated by the start date of the thread they reside in.

Option: Username
Result: Results are sorted alphabetically by the username of the poster

Option: Forum
Result: Results are organised and grouped by which forum they appear in

Show Results As
Here you can select whether the results are displayed as a list of threads (i.e. the complete thread with the found post inside), or as individual posts (which shows a small overview of the post and a direct link to view the full post).

Search In Forums
If you only want to perform your search in a particular forum, select it on the list here. To select multiple forums, hold down ctrl on the keyboard and click each forum. The "Search Child Forums" checkbox searches through subforums of the selected item - so ticking the box and highlighting General will search General Chat, Muppet Corner, Projects and Featured Car Of The Month. To search all forums, highlight the Search All Open Forums option.