An element is the html tag plus the content plus the closing tag. It's a chunk of code and content. "Block" means that it takes up the whole width of the screen (the height is usually determined by the content) and has a line break at the end (by default). Block elements also can have padding, nest other block elements. Below are some.
Headers create hierarchy to the web page. If you had a resumé for example, your name would be surrounded in a h1 tag (tags start with < and end with >), terms like education, work experience, etc. would be h2s and if further categories were needed, you'd use h3, h4, h5 and h6.
That was fun. But you wouldn't use headers in that way. Below you can see an h2 with h3s beneath.
Ordered lists put a number in front of each item.
Pretty common for navigation or most any content that sits in parallel.
We've been using paragraph tags all along.