呃。。。这个会不会太基础了。最近在复习，所以基础知识也不能够忽略。
根据HTML 4.01 规范，其描述如下（http://www.w3.org/TR/html401/struct/global.html#h-7.5.3）：
1. 内容模型：block元素可以包含block元素和inline元素，inline元素可以包含数据和其他inline元素（就是说inline不能包含block）。核心思想就是相比inline元素，block元素构建的结构都“比较大”。
2. 格式：默认，block元素之间有换行，inline元素之间没有。
3. 文本方向：在实现文本方向上，block元素和inline元素有差别。block元素会继承dir属性，inline不会。
原文：
7.5.3 Block-level and inline elements
Certain HTML elements that may appear in BODY are said to be "block-level" while others are "inline" (also known as "text level"). The distinction is founded on several notions:
- Content model
- Generally, block-level elements may contain inline elements and other block-level elements. Generally, inline elements may contain only data and other inline elements. Inherent in this structural distinction is the idea that block elements create "larger" structures than inline elements.
- Formatting
- By default, block-level elements are formatted differently than inline elements. Generally, block-level elements begin on new lines, inline elements do not. For information about white space, line breaks, and block formatting, please consult the section on text.
- Directionality
- For technical reasons involving the [UNICODE] bidirectional text algorithm, block-level and inline elements differ in how they inherit directionality information. For details, see the section on inheritance of text direction.
Style sheets provide the means to specify the rendering of arbitrary elements, including whether an element is rendered as block or inline. In some cases, such as an inline style for list elements, this may be appropriate, but generally speaking, authors are discouraged from overriding the conventional interpretation of HTML elements in this way.
The alteration of the traditional presentation idioms for block level and inline elements also has an impact on the bidirectional text algorithm. See the section on the effect of style sheets on bidirectionality for more information.