Permalinks are basically Permanent links of webpage or posts.This Links are url ,which gives unique identity to your post or page and make it easier for the non-techinal to remember your page or page name.
Types of Permalinks:
1) Ugly: These are by default permalink with page id like http://abc.com/?p=N (where N is id).
2) mod_rewrite: These are mostly used permalink ,as it is very much user friendly like http://iblogtechs.com/2015/06/19/permalinks/ .It is easy to remember and use.
3) Pathnifo: It is almost same as mod_rewrite ,with the only difference that it has prefixed index.php in the url like http://iblogtechs.com/index.php/2015/06/19/permalinks/ .