Font-editing programs are the tools that type designers use to create fonts from scratch. They include tools to draw character outlines as well as to edit those outlines later. Font editors can be useful to the nondesigner as well, as they can add characters to a font (a digitized corporate logo, for example). They are also sometimes used by demanding typographers to improve the quality of the kerning information within a font.
Creating customized fonts can have its advantages, but its disadvantages are very serious. Edited fonts become unique fonts, and unless they have been given unique names, they can become confused with existing retail fonts. Fonts that have had their kerning information altered, for example, will cause text to compose in a unique way, and these differences are enshrined in the font, not in the document. If that document travels to a place where the custom font that created it is unavailable, it will not compose correctly, line endings will change, and whole layouts can become disrupted.
Because font files are mobile, a customized font that escapes its handlers can cause havoc if it becomes confused (or used) with its unedited forebear. Edited fonts, then, have to be employed with great care and control.