A partnership is when two or more people work together to complete a task. Those people are partners. Partner may share a business together for the profit of each other. If one business partner ends up making money they both share the profit. Some partners are not individuals but corporations or other groups.
Partnerships may be temporary, e.g. dancing partners (two people who dance together) or partners at a card games such as bridge in which partners work together against the other partners (their opponents).
A partner may also be somebody who lives with another person, e.g. a husband or wife (marriage partners), or a very good friend who shares his or her life with the other person. The word partner in this sense is used a lot in modern times because it does not discriminate between married or unmarried couples.