The obvious resource is MSDN - There is a section entitled Planning for High Activity that covers most of the concepts and will give you the right terminology to then go looking for other resources on the web. As with a lot of Microsoft server products, MSDN also has lots of white papers covering specific BizTalk scenarios.
Most good BizTalk books also include a section on load balancing concepts (Professional BizTalk Server 2006 has an example).
Beyond that, there are several key concepts that you may find helpful, particularly around the use of terminology (some of BizTalk's usage can be misleading).
Load Balancing
BizTalk Server is, by the nature of its architecture, load balancing. What that means is that if you have more than one BizTalk Host connecting to a MessageBox database, the messages within the database will be spread evenly across the hosts participating in the BizTalk group. (with caveats around just what BizTalk processess have been configured to run in each Host).
There is also the concept of Network Load Balancing which is Microsoft Network Load Balancing Services or any equivalent service. In BizTalk this applies at the web level, for receive adapters using the HTTP protocol (e.g. the HTTP adapter, the SOAP adapter and WCF HTTP adapters). This load balancing is not actually a BizTalk service but is instead a load balancing layer provided on top of the BizTalk isolated host adapters to ensure high availability of the web resources. It is configured the same as any other NLB service.
Clustering
When clustering is mentioned in BizTalk it is used to refer to one of two things - clustering at the SQL layer to provide high availability and failover, and BizTalk Host Clustering.
SQL Clustering - this is simply (though it isn't simple to do, just say) a matter of providing a SQL server cluster that runs the BizTalk server databases, allowing for database failover. This is not a BizTalk specific technology.
BizTalk Host clustering - in this case a BizTalk Server Host is marked as being clustered when creating it inside BizTalk. This is a BizTalk specific setting that essentially states that one and only one instance of the host will be running at a time, and that by extension all the resources within this host will also only have a single instance. It is primarily intended for usage for adapters like the FTP and MSMQ adapters that behave incorrectly when more that one is allowed to run at the same time.
This edit is in response to the OP's comment asking for further details. Hopefully this make things clearer. If you have more questions about specifics I can possibly answer them but this pretty much exhausts my theory knowledge about high availability environment configuration. I'm primarily a BizTalk dev and solution designer, when it comes to network intricacies there are people where I work who fill in the nitty gritty detail and implementation of these designs.
Network Load Balancing for HTTP Based Adapters
The key point I was trying to express here was that Network Load Balancing in the context of BizTalk is no different for any other Network Load Balancing scenario.
BizTalk has two type of hosts, In Process and Isolated. In Process hosts are individual BizTalk services running on servers (with one host instance per server). Isolated hosts are actually delegates to a web server (IIS) that handle all HTTP based adapters (the HTTP adapter and the SOAP adapter plus certain configurations for the WCF adapter)
When you introduce Network Load Balancing to a BizTalk environment what you are doing is intoducing it at the web server layer, for the Isolated host hosted adapters.
Here is the MSDN page for the introduction to NLB. One of the key points about NLB is expressed in the page in the following quote:
Network Load Balancing allows all of
the computers in the cluster to be
addressed by the same set of cluster
IP addresses (but also maintains their
existing unique, dedicated IP
addresses).
By setting up NLB you allow multiple isolated host servers to handle internet traffic directed at a single dedicated IP address. The NLB configuration farms out the work.
Clustering BizTalk Adapter Handlers
In my answer above I stated that certain BizTalk adapters behave incorrectly when allowed to run within multiple BizTalk Host Instances. This is very adapter specific in terms of the why, so the best expansion on that answer I can give is the following quote from the MSDN documentation, dealing with the FTP adapter specifically.
For most of the BizTalk integrated
adapters, high availability can be
achieved by creating multiple adapter
handlers to run on BizTalk host
instances on different BizTalk servers
within a BizTalk group. FTP adapter
receive handlers should not, however,
be configured to run in multiple
BizTalk host instances simultaneously.
This recommendation is made because
the FTP receive adapter uses the FTP
protocol to retrieve files from the
target system and the FTP protocol
does not lock files to ensure that
multiple copies of the same file are
not retrieved simultaneously when
running multiple instances of the FTP
receive adapter.
As they say, the FTP adapter utilises the FTP protocol which does not lock files. Because BizTalk is natively a highly parallel system, if you allowed multiple BizTalk hosts to host an instance of the FTP adapter you would end up with multiple copies of the same FTP message received into your BizTalk system. What BizTalk clustering does is ensure that any clustered BizTalk hosts will run on 1 and only 1 host instance. By placing your FTP receive handler inside a clusterd host, you ensure that:
- you will always have an FTP adapter running so long as a BizTalk host is running
- you will never have more than one FTP adapter running.
Additionally you can use a BizTalk clustered host to reduce load on a system. For example, a BizTalk SQL adapter receive location that has been configured to poll, will poll on all host instances. While this would not necessarily cause multiple message instances, it could cause undue load on the SQL server you poll, or even create deadlock scenarios depending on the design of the called stored procedure, so clustering the SQL Adapter receive handler can be a good idea.