what is difference between Two-Tier and Three-Tier Architecture?

Vithal, Please respect all seniors here. if SA says something there must be some reason. it's like pain when someone downvote us. But in long run it will help us to improve our skills(softskills). See my solution for more information on your question.

Dont be sorry. we all are working here as friend. people here are contributing when they have extra time. So we should respect their time and we should be down to earth.

Those are golden words, Amit; I totally agree and support this view.

I'm not sure you had to protect me against the negative comment by Vithal, especially if you use such shaky arguments as who is "senior", etc. Thank you very much for your support, anyway. We all should respect each other and our rightfulness or failure should only be measured by truth, nothing else. I saw his comment; he complains that I evaluated his posts negatively and maintains that his question is correct.

Every concern deserves some explanation. I don't remember all my comments, but the chances are, that I pointed out incorrectness of questions, so OP might think about: who is really responsible for incorrectness of the questions. So, I can give my explanation about the present question. It's two-fold.

The title has the form "what's the difference between {0} and {1}"? It cannot be a correct question. I answered: "one has three tiers, another -- two tiers". It means that nearly anything can be called a "difference". This is just a figure of human speech, not a strict concept. As I usually ask: can you tell the difference between apple and Apple?

Now, similar thing goes about what is "best". Logically, to answer this question, one would need a definition of "better than". For architectures, it is really absolutely not defined. Let's look at this in the following way: if, say, some N-tier architecture for some fixed N existed as was "the best", all n-tier architectures for n!=N would never be used. Nevertheless, different number of tires are used in different systems. This is because there is nothing special about any particular number of tiers and their roles: they are devised to solve some practically important situations. So, n-tier architectures does not have any absolute value; their value is totally related to some sets of application problems they may or may not be well applicable to, no more. So, asking "what's the best" makes no sense at all and cannot be answered.

For an enquirer, it's way to easy to play offended card and pretend the criticizing person is responsible for some trouble. But in this case, who is the looser? :-)
For everyone, it's much more fruitful to listen to criticism and try to improve one's work by actually using this criticism. Of course, anyone has a right to ask for proper arguments. I personally would gladly answer to such concerns.

Thank you.
--SA