Think of these guys as poetic paragraphs. A stanza is a division within a poem where a group of lines are formed into a unit. The word stanza comes from the Italian word for "room." Just like a room, a poetic stanza is set apart on a page by four "walls" of blank, white space.
A poem can have regular stanzas, such as Keats's "La Belle Dame Sans Merci," which is made up entirely of quatrains. Or it can have stanzas of all shapes and sizes, like those you might see in a free verse poem like Langston Hughes's "The Negro Speaks of Rivers."