BestPractices are practices you should generally follow, unless you really have reason not to. Often, these have crystallised over time, as experience with a particular technology accumulated. Sometimes, you may wish to forgo them, such as when they are more work than you want to invest and their benefits are irrelevant to your goals. However, they usually cover a lot of contingencies you may not foresee; ignore them at your own peril, as the saying goes.
This wiki has a collection of documents that attempt to record the best practices for setting up servers and services in HowTo style. They are written with the assumption that you understand the service; you should not go looking for introductory material here. Another assumption is that you've set up related services as outlined in their own best practices pages. For instance, SMTPBestPractices may assume that you have LDAP configured as documented in LDAPBestPractices.
For general notes about a particular service look for a ServiceNotes page; for an introduction to setting it up look for a ServiceHowTo page.
There are some other websites which discuss similar topics:
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