HTML elements
If you don't know what an element is or how you must use it, I recommend you read the "HTML tags and attributes" tutorial that you can find in the HTML tutorials section.
Below is a list of all elements in the HTML standard up to version 5, along with a short description about it's use/purpose. Clicking on any element on this list will take you to a page with a detailed description about it, some examples of its use and a list of all attributes it supports.
You should bear in mind that newer elements, like those added in HTML5, may have partial support or no support at all in some browsers. This will change with time, but meanwhile, if you're designing a website that needs to be viewed correctly in all browsers, you may want to avoid their use or replace them with other alternatives like scripts. All features of HTML5 with incomplete browser support are properly marked with red and accompanied by warning messages.