title_body: What are serif and sans serif fonts on different ebook reading systems?
upvoted_answer: Kobo ereader devices
  
  My 1st gen Kobo Glo has the following fonts pre-installed (I suppose other Kobo devices use the same set; it is possible for the user to add more but I think this is beyond the scope of your question):
  
  Serif: (1)
  
  
  Amasis
  Caecilia
  Georgia (Default Serif font; also default font if nothing is specified in the ebook)
  Kobo Nickel
  Malabar
  
  
  Sans Serif:
  
  
  Avenir Next (Default Sans Serif font)
  Gill Sans
  
  
  Other:
  
  
  KBJ-TsukuMin Pr6N RB (Serif) (2)
  KBJ-UDKakugo Pr6N M (Sans Serif) (2)
  OpenDyslexic
  
  
  Of course, these customizations are optional, and it is possible to default fonts as defined inside the ebook.  
  
  About titles, if there are not specific settings inside the ebook stylesheets, they are treated like other text: besides having a larger size, if not explicitly set to something else, the default Georgia is used.
  
  Kobo app for Android
  
  Compared to the ereaders, the app is more limited in the customization options:
  
  
  Default settings
  Droid Serif
  Droid Sans
  
  
  
  
  1. Including slab serif
  2. I'm not really sure but I think that these are used for Chinese/Japanese languages compatibility.