阅读Scott Hanselman关于这个问题的帖子:关于“三/多层/层架构/设计”的提醒:

请记住，在“Scott的世界”(希望也是你的世界:)中，“Tier”是一个部署单元，而“Layer”是代码中责任的逻辑分离。你可能会说你有一个“三层”系统，但在一台笔记本电脑上运行它。你可能会说你有一个“三层”系统，但只有ASP。与数据库对话的NET页。精准就是力量，朋友们。

IBM的三层体系结构文章有一个章节专门讨论这个主题:

In discussions of three-tier architecture, layer is often used interchangeably – and mistakenly – for tier, as in 'presentation layer' or 'business logic layer.' They aren't the same. A 'layer' refers to a functional division of the software, but a 'tier' refers to a functional division of the software that runs on infrastructure separate from the other divisions. The Contacts app on your phone, for example, is a three-layer application, but a single-tier application, because all three layers run on your phone. The difference is important, because layers can't offer the same benefits as tiers.

我发现一个定义说，层是逻辑分离，层是物理分离。

Logical layers are merely a way of organizing your code. Typical layers include Presentation, Business and Data – the same as the traditional 3-tier model. But when we’re talking about layers, we’re only talking about logical organization of code. In no way is it implied that these layers might run on different computers or in different processes on a single computer or even in a single process on a single computer. All we are doing is discussing a way of organizing a code into a set of layers defined by specific function. Physical tiers however, are only about where the code runs. Specifically, tiers are places where layers are deployed and where layers run. In other words, tiers are the physical deployment of layers.

来源:Rockford Lhotka，所有应用程序都应该是n层吗?

我使用层来描述我的解决方案组件中的架构师或技术堆栈。通常在涉及网络或进程间通信时，我使用层对这些组件进行逻辑分组。

层是应用程序内相关功能[代码]的逻辑分离，层之间的通信是显式和松散耦合的。[表示逻辑，应用逻辑，数据访问逻辑]

层是在单个计算机(进程)中[托管在单个服务器上]的层的物理分离。

如图所示:

1-Tier & 3-Layers « App Logic  with out DB access store data in a files.
2-Tier & 3-Layers « App Logic & DataStorage-box.
2-Tier & 2-Layers « Browser View[php] & DataStorage[procedures]
2-Tier & 1-Layers « Browser View[php] & DataStorage, query sending is common.
3-Tier & n-Layer  « Browser View[php], App Logic[jsp], DataStorage

n层优势: 更好的安全性 可伸缩性:随着组织的发展，您可以使用DB-Clustering扩展您的DB-Tier，而不涉及其他层。 可维护性:Web设计人员可以更改视图代码，而无需触及其他层上的其他层。 轻松升级或增强[例如:您可以添加额外的应用程序代码，升级存储区域，甚至为移动，平板电脑，pc等单独的设备添加多个显示层]

来自博客的图表