You can think of a plain document as a single column of text stretching the width of the page: when a line of text reaches the right border of the page it "wraps" around onto the next line, before eventually moving to the next page.
When working with two or more columns, the text wraps to the next column and only wraps to the next line at the end of the last column. Otherwise, all the text and paragraph formatting remains the same.
You can apply create up to six columns for individual paragraphs or for the entire document - see Creating columns.
Columns are, by default, of equal width. You can set the width of all the columns, and all the inter-column gaps, in one go or else have unequal column widths - see Formatting columns.
To see how to remove and split columns and how columns can be used in conjunction with frames, see Working with columns.