Affiliations are agreements between wikis to promote and support each other. Usually this is done by each wiki having a link to the other on their main page.
Wikis that affiliate usually have similar topics or another connection. For example, both might be anime wikis, or be about movies that have the same director.
By their nature, affiliations are always mutual. They involve two or more wikis who agree to affiliate and agree on the conditions of that affiliation. Communities should not claim to be affiliated with another wiki without the agreement of its admins and/or community.
If there is no agreement, one wiki may still link to another, but they should not claim it is an affiliation. Instead they might have a section on their main page for "Recommended Wikis" or "Other Wikis About [linking topic]".
Affiliated wikis often display each other's logos on their main pages. Logos are often very personal to a wiki's community and should not be used on other wikis without permission.
For wikis covering the same topic in a different language, interlanguage links can be used instead.