This topology uses an enterprise services farm that delivers enterprise-wide services to other farms within the company. Typical services are the user profile service, managed metadata and of course search. The colored lines between the farms point to the consumed services in another farm.
The other farms consume services from the enterprise services farm in addition to some local services. In this example, the 4 other farms are split up by the type of sites they provide: publishing sites, collaboration sites, my sites and departmental sites.
Be aware of the fact that consuming and publishing these services does not imply a strict hierarchy. Each farm in the topology can be used to publish services to other farms and each farm can consume services at the same time.
More info about these principles can be found here: