A tributary is a stream or river which flows into a mainstem (or parent) river. A tributary does not flow directly into a sea. Tributaries and the mainstem river serve to drain the surrounding drainage basin of its surface water and groundwater by leading all the water out into an ocean or some other large body of water.
A confluence is when two or more bodies of water meet together, usually referring to tributaries. An affluent is synonymous to the word 'tributary', being defined as a stream or river that simply flows into a larger one.
The opposite of a tributary is a distributary, a river that flows away from the main stream.
A 'right tributary' and 'left tributary' are terms stating the relative positions of the tributary to the mainstem river. These terms are applied from the perspective of looking downstream (in the direction the current of the water is going).
The Strahler Stream Order examines the arrangement of tributaries in a hierarchy of first, second, third, and higher orders, with the first order tributary being typically the least in size. For example, a second order tributary would compose of two or more first order tributaries combining to form the second order tributary.