It's very simple you can just Google it out : Sitemaps, as the name implies, are just a map of your site - i.e. on one single page you show the structure of your site, its sections, the links between them, etc. Sitemaps make navigating your site easier and having an updated sitemap on your site is good both for your users and for search engines. Sitemaps are an important way of communication with search engines. While in robots.txt you tell search engines which parts of your site to exclude from indexing, in your site map you tell search engines where you'd like them to go. You can find more detailed information from Google:
robot.txt you need to use if you want to disallow search engines indexing some pages or sections of your website. It may be necessary to avoid duplication of information and not to fall under sanctions of Google Panda
I will explain you in very simple terms sitemap creation and submitting it to webmaster tools will help you index your website urls in that search engine. Robot.txt we can include for any page, part of page or images which we think is not to be get crawled by search engine crawlers.