Agile Model is useful for continous interaction with the
customer ,in this model the Tesing phaze will start first
before developing.

Advantages: - Testing and Develeoping will be done
simultaneously , There is continous intercation with the
custonmer.

Disadvantages: - Cost is the main criteria here,

In future many companies will follow this model if cost is
not main criteria

If you would like to follow a method which should be 
feasible for the sudden changes in the requirements, AGILE 
Method is a best choice. I agree with Mr. Narendra, that 
this method will give you room to work very closely with 
CLIENT or CUSTOMER. 

SCRUM Model is the Agile Methodology. SCRUM is a huge 
topic. Hope this space is not enough to discuss. To make it 
short, project will be devided in to number of Sprint. Each 
Sprint would be 2 weeks, 4 weeks, 6 weeks or 8 weeks, 
depends on their project specifications. All the 
requirement are placed in to a document called Product 
Backlog with the priority of implementation. Every sprint 
start there will be a meeting called Start-Up meeting. In 
this meeting backlog items will be dragged in to the 
Sprint. Based on implementation order development will 
progress and parallel to that testing will be taken place. 
In the startup meeting Development and Testing team both 
will sit together for the each story estimation. and so 
on ...I am tired