What architecture and design pattern to use for a asp.net web application

In addition to the excellent links already posted:

Architecture is a somewhat nebulous concept, but relates to high level design decisions, and things which become expensive to change later on. The '3 tier' architecture is traditional for web applications and it works well (though one might argue that a modern web app is actually 4 tier: database, business logic, server-side presentation, and client-side presentation i.e. Ajax/Javascript).

Design patterns are essentially ideas that can be re-used in different scenarios. It's normally used for lower level design, on the code or module level – for example MVC (and MVP and MVVM and any other variants), publish-subscribe, enumeration, etc. These sometimes get built into the language or at least the language community; for example the way .Net and Java do collections is essentially the formalisation of the patterns C++ programmers were using for them.

I've not heard the term 'architectural pattern' before but it is a reusable architectural design – so 3-tier, event driven, plugin-based and any other common architecture would qualify.

The important thing about patterns is that they're just names for particular groups of ideas. Don't use a pattern just because it's a pattern, use it if the ideas it encapsulates are appropriate for your project.

Have a look at the 3-tier and design pattern definitions here[^].
MVC is a good design pattern to use in ASP.Net.

Not much to add to the explanations so far. Patterns in general are a great way to make use of expert knowledge (that is solution blueprints for often occuring problems). It's definitely worth it to get familiar with at least the main design patterns.

Another good page which explains the basic patterns very well is:
http://sourcemaking.com/[^]

If you're in an ASP.NET environment i would forward you to the MS Application Archhitecture Guide. It epxplains the main architecture types (not only 3- Tier) very well in my opinion. It also includes plenty of links to HowTo's for specific tasks in ASP.NET and many more stuff.

http://msdn.microsoft.com/en-us/library/ff650706.aspx[^]