A very detailed blog post on process of tracking by Thiago Almeida can be found here:
Other useful blog posts on tracking:
BizTalk Server is optimized for throughput, so the main orchestration and messaging engines do not actually move messages directly to the BizTalk tracking or BAM databases, as this would divert these engines from their primary job of executing business processes.
Instead, BizTalk Server leaves the messages in the MessageBox database and marks them as requiring a move to the BizTalk Tracking database. A background process (the
tracking host) then moves the messages to the BizTalk Tracking and BAM databases. It is recommended to create a dedicated host for tracking. See MSDN
Configuring a Dedicated Tracking Host.
Another important place to find a huge amount of BizTalk related articles is the TechNet Wiki itself. The best entry point is
BizTalk Server Resources on the TechNet Wiki.