Host is nothing but the logical container of host instance.
from which we can create host instance.

Host instance is a Win-NT service.

A BizTalk Server Host is a logical set of zero or more 
BizTalk Server run-time processes in which you deploy items 
such as adapter handlers, receive locations (including 
pipelines), and orchestrations.

A host instance is the process where the message 
processing, receiving, and transmitting occurs You install 
a host instance on each server running BizTalk Server 2006 
that has one or more hosts mapped to that server

If the application using it is not too large, the entire 
BizTalk Server 2004 engine can be installed on a single 
computer. In many situations, however, this is not the 
right solution. The number of messages the engine must 
handle might be too great for one computer, or redundancy 
might be required to make the system more reliable. To meet 
requirements like these, the BizTalk Server 2004 engine can 
be deployed in a number of ways.
A fundamental concept for deploying the engine is the idea 
of a host. A host can contain various things, including 
orchestrations, adapters, and pipelines. Hosts are just 
logical constructs; to use them, a BizTalk Server 2004 
administrator must cause actual host instances to be 
created. Each host instance is a Windows process, and as 
the following figure shows, it can contain various elements.
Figure 6 Host computers
 
In this figure:
•	Computer A runs two host instances. One contains a 
receive adapter and receive pipeline, while the other 
contains the orchestrations P and Q. 
•	Computer B runs just one host instance, also 
containing the orchestrations P and Q. 
•	Computer C, like computer A, runs two host 
instances, but neither of them contains an orchestration. 
Instead, each of these instances contains a send pipeline 
and send adapter. 
•	Computer D houses the MessageBox database that is 
used by all of the host instances in this configuration.
This example illustrates several ways in which hosts might 
be used. For example, because both computers A and B run 
host instances that contain the orchestrations P and Q, 
BizTalk Server 2004 can automatically assign requests to 
these orchestrations based on the availability and current 
load on each computer. This allows a business process to 
scale up as needed for high-volume applications. Notice 
also that computer C contains two different ways to handle 
outgoing messages. One way might rely on a standard BizTalk 
Server 2004 adapter, such as the HTTP adapter, while the 
other might use a custom adapter to communicate with a 
particular application. Grouping all output processing on a 
single computer like this can make good sense in some 
situations. And because each host instance is isolated from 
every other host instance—they are different processes—it 
is safer to run code that is not completely trusted, such 
as a new custom adapter, in a separate instance. It is also 
worth pointing out that even though this example contains 
only a single instance of the MessageBox database, you can 
also replicate or cluster the databases to avoid creating a 
single point of failure.
Managing Applications
The BizTalk Server 2004 engine provides a range of 
services, and several different tools are used to manage 
this environment. The primary tool is the BizTalk 
Administration console, a Microsoft Management Console 
(MMC) snap-in. This tool enables you to create hosts, 
assign hosts to computers, start and stop orchestrations, 
and perform many other administration tasks. You can even 
dynamically add computers and specify what hosts should be 
assigned to them while an application is running—there is 
no need to shut the application down to make these changes. 
You can also access the functions of the Administration 
console programmatically through Windows Management 
Instrumentation (WMI), which enables you to create scripts 
that automate management functions.