So another one in my “What Every SharePoint Consultant Should Know” series, this one about basic server roles. This really isnt my strength so I thought that this might help me and hopefully you.
Web Front End (WFE)
What does it do?
Contains IIS and is responsible for rendering pages to users. Contains the Web Application that distributes content to users.This involves fetching content from the SharePoint databases and “look and feel” from the database and file system and then rendering, caching (depending on settings), and returning the page to the user.
How do I configure it?
When installing SharePoint on a server you can choose “WFE Only” but keep in mind that this will only allow you to run a subset of services on the server.
When would I use it?
When you install SharePoint at least one server needs to be a WFE/Application Role. So you need one server at least in the farm to have this role, you have no choice.
Index Role
What does it do?
This role is responsible for crawling content and building up an index. Now you may think that the index server crawls all the content but this is NOT the case. The Index roles managers the crawl, performs crawls for items external to the fame (File Shares, Web Sites, BDC) but makes requests to Query or WFE’s to crawl internal content.
How do I configure it?
If you choose the Search Indexing role you can create a query server by simply check the “Use this server for indexing content” option and uncheck the “Use this server for serving search queries” option (the opposite of creating an Query role)
When would I use it?
This is a very intensive process and it is suggested that you put this on a separate server if you can, in many cases this is the first role to offload.
Any thing else I should know?
Some people create a WFE/Index role on a box for a couple of reasons which make a lot of sense, see this post from Joel Oleson which explains it in more detail. As mentioned above if you put he WFE/Index role on the same box the server won’t need to request content from other WFE’s or Query servers and can crawl local content locally, thereby reducing bandwidth.
Another thing is that the Query/Index role cannot be split on a WSS farm, you only get the Search role which runs both services.
Query Role
What does it do?
This role is responsible for taking users search queries and performing keyword searches on the local copy of the index that it stores and passing metadata queries to the database to return results back to users.
How do I configure it?
If you choose the Search Indexing role you can create a query server by simply unchecking the “Use this server for indexing content” option and checking the “Use this server for serving search queries” option.
When would I use it?
If you have a high number of search queries being performed then consider having the query role as a separate server.
Any thing else I should know?
Yes!! The infamous Query/Index problem!!!! If you have ANY server on a farm with BOTH the query and index roles on the same server the index WILL NOT get propagated to other query servers. SharePoint assumes that if the roles are on the same server that you know what you are doing and dont need to propagate the index. You have been warned 
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