Because traffic can come from multiple sources, identifying the source of an inbound visitor is extremely important. The can enable an online marketer to identify the source of traffic which generates the most valuable conversion actions, and can improve the number and quality of conversion actions.
In its simplest form, when publishing links on different sites, the source of a link can be identified as such:
Advertisement on Google Adwords:
Advertisement on Microsoft AdCenter:
Advertisement on Facebook:
You'll note that the term crsource is present in the query string of the inbound link.