Sitemaps are much-hyped and oft-misunderstood. Most people have heard they need a sitemap so Google will like their site but how to go about creating one?
Sitemaps are recommended for search engine optimization to help Google (and other search engines) discover all of the content on your site. They are especially helpful when your site is large or has a complex structure which might make it harder for Google to find all the pages on your site.
Sitemaps come in two flavours: XML and HTML
XML sitemaps are designed for search engine bots, not humans.
HTML sitemaps are more geared toward humans, but search engine bots can crawl them too.
How To Create An XML Sitemap
If you use the WordPress SEO plugin by Yoast, it has the capability to create an XML sitemap. If you are not using that plugin, or are an advanced user who needs even more fine-grained control than that which the Yoast plugin provides, I recommend BWP XML Sitemaps.