Relationships and their reciprocal types are shown between terms of the same ontology.
The two major relationship types used in all ontologies are:

is part of has part	Term B is a part of term A during the time at which they both exist. Term A may have a part B during the time at which they both exist. leaf is part of: tree tree has part: leaf, trunk, root
has type is a type of	Term B is a type of term A. Term A has subtypes B and C. parrot is a type of: bird bird has type: parrot, eagle, pigeon

The anatomical ontology (AO) uses another relationship type indicating how structures develop from another one in time:

develops from develops into	Structure A develops from structure B. Structure B develops into structure A. There can be a one to one, one to many, or many to one correspondence. For example, structure A develops from structure B and C, or structure A develops into structure B and C. cheese develops from: milk, bacteria milk develops into: cheese, yogurt, ice cream

These relationships are representations of formal types described by the OBO-REL ontology: http://www.bioontology.org/wiki/index.php/RO:Main_Page