case / number

Russian noun endings change according to number and case. Вода is the nominative form. Воды can be the nominative plural form, the genitive singular form or the accusative plural form...It depends on its sentence position. Russian has a case system, and it's necessary to read about it to understand what's going on.

Вода. Base form of the word. "This is a water" (Это вода) Воды. For example: near water (Около воды), a lot of water (много воды) or plural (waters) Maybe forgot something :)