In the Character dialog, you can select different font types when inserting a character. If you insert a character with "(normal text)" selected in the Font field, that character will use the same font properties of the content where it is being inserted. But if you select a specific font in the Font field, the character will instead use those properties at the point of insertion, which will result in a <span> tag being created.
A <span> tag is used in stylesheets to control the look of inline text (content at the character level).