<main> is for content unique to this page. Use <main> only once per page, and put it directly inside <body>. Ideally this shouldn't be nested within other elements.
<article> encloses a block of related content that makes sense on its own without the rest of the page (e.g., a single blog post).
<section> is similar to <article>, but it is more for grouping together a single part of the page that constitutes one single piece of functionality (e.g., a mini map, or a set of article headlines and summaries), or a theme. It's considered best practice to begin each section with a heading; also note that you can break <article>s up into different <section>s, or <section>s up into different <article>s, depending on the context.
<aside> contains content that is not directly related to the main content but can provide additional information indirectly related to it (glossary entries, author biography, related links, etc.).
<header> represents a group of introductory content. If it is a child of <body> it defines the global header of a webpage, but if it's a child of an <article> or <section> it defines a specific header for that section (try not to confuse this with titles and headings).
<nav> contains the main navigation functionality for the page. Secondary links, etc., would not go in the navigation.
<footer> represents a group of end content for a page.
<span> is an inline non-semantic element, which you should only use if you can't think of a better semantic text element to wrap your content, or don't want to add any specific meaning.
<div> is a block level non-semantic element, which you should only use if you can't think of a better semantic block element to use, or don't want to add any specific meaning.
