HTML, or HyperText Markup Language, is basically the language that dictates what shows up on a webpage. CSS, or Cascading Style Sheet, on the other hand dictates how it shows up.
For instance, you can use HTML to create some text that's on a page (what). You use CSS to make add a thick purple, dashed box around the text (how it appears).
HTML is composed of elements, typically made up of tags. Tags are those little pairs of brackets, such as <body>, <img> <div> and so forth. Typically tags come in pairs, an opening and closing tag. Opening tags denote the start of an element, such as <h1>. Closing tags denote the end of an element, and are differentiated by a backslash </h1>.
CSS enables you to describe precisely how you want each element on your website or blog to appear, in one succinct spot. When you make a change to the CSS, it cascades and changes each and every iteration of that element on your blog or website. For example, rather than having to uniquely code the font for each of your post titles, CSS dictates it automatically each time you add a new post.
It's handy to have an understanding of both, however HTML is definitely the more powerful language when blogging. HTML allows you to create and edit widgets, easily modify and troubleshoot post content, and more.
I'll be diving into how to accomplish specific tasks with CSS and HTML in future posts, like how to create a hyperlinked signature for commenting on other blogs, creating custom social icons, changing margins, widths, and spacing of your layout, and more. If there's a specific HTML tidbit you're looking to learn, let me know and I'll see what I can do to cover it :)