This is an adverb, used to link items in a sentence or to make links between sentences. For example:The oxygen they produce benefits the fish, and they also provide a source of food and shelter. It is frequently used with and or but, but it is not a conjunction. In formal writing also should not be used instead of and:The closet is larger than a pit and therefore lasts longer also it is easy to empty. Here the speaker needs to add and (→ and also), or replace also with and.