SEO ( Search Engine Optimizing ) is for Websites.
ASO ( App Store Optimizing ) is for Mobile Apps.
So lets elaborate.
SEO is manipulating search algorithm to get better ranks for websites , used by the web developers and web marketers to optimize web site to increase search visibility (SERP) over various search engines like Google ,Yahoo , Bing etc . SEO is the easy way to get more organic traffic without spending a lot for ads. SEO include Backlinks,Blog Posting,Social Media Bookmarks etc
ASO is used to increase rank over app stores ( Google play store , Apple iTunes , Windows stores ) like if you search for ‘ Video Player ‘ #1 , #2 & #3 positioned app will have better chance to get downloaded by the users. Methods of doing ASO are Title Optimization , Descrption optimization , Screenshot optimization , Video etc.