HTML tags come in two varieties; block and inline. Block elements always start a new line for themselves, and any content that follows them also starts on a new line. <p> and <h1> are familiar block level elements. Inline elements follow each other like the letters in a sentence, only beginning a new line when the run out of space on the previous one. Common examples are they <em> and <strong> tags.
The <div> and <span> tags can be used to create block and inline elements respectively. They are used to help layout a page.
The flow in an HTML document is from top left to bottom right. Starting in the top-left hand corner of the documents each one will appear to the right of the previous one until there is no more space on the line, at which point the next object will go at the leftmost end of the next line.