Software Development Life Cycle (SDLC)
This is also known as Classic Life Cycle Model (or) Linear 
Sequential Model (or) Waterfall Method. This model has the 
following activities. 

1. System/Information Engineering and Modeling

As software is always of a large system (or business), work 
begins by establishing the requirements for all system 
elements and then allocating some subset of these 
requirements to software. This system view is essential 
when the software must interface with other elements such 
as hardware, people and other resources. System is the 
basic and very critical requirement for the existence of 
software in any entity. So if the system is not in place, 
the system should be engineered and put in place. In some 
cases, to extract the maximum output, the system should be 
re-engineered and spruced up. Once the ideal system is 
engineered or tuned, the development team studies the 
software requirement for the system. 

2. Software Requirement Analysis

This process is also known as feasibility study. In this 
phase, the development team visits the customer and studies 
their system. They investigate the need for possible 
software automation in the given system. By the end of the 
feasibility study, the team furnishes a document that holds 
the different specific recommendations for the candidate 
system. It also includes the personnel assignments, costs, 
project schedule, target dates etc.... The requirement 
gathering process is intensified and focussed specially on 
software. To understand the nature of the program(s) to be 
built, the system engineer or "Analyst" must understand the 
information domain for the software, as well as required 
function, behavior, performance and interfacing. The 
essential purpose of this phase is to find the need and to 
define the problem that needs to be solved .

3. System Analysis and Design

In this phase, the software development process, the 
software's overall structure and its nuances are defined. 
In terms of the client/server technology, the number of 
tiers needed for the package architecture, the database 
design, the data structure design etc... are all defined in 
this phase. A software development model is thus created. 
Analysis and Design are very crucial in the whole 
development cycle. Any glitch in the design phase could be 
very expensive to solve in the later stage of the software 
development. Much care is taken during this phase. The 
logical system of the product is developed in this phase.

4. Code Generation

The design must be translated into a machine-readable form. 
The code generation step performs this task. If the design 
is performed in a detailed manner, code generation can be 
accomplished without much complication. Programming tools 
like compilers, interpreters, debuggers etc... are used to 
generate the code. Different high level programming 
languages like C, C++, Pascal, Java are used for coding. 
With respect to the type of application, the right 
programming language is chosen. 

5. Testing

Once the code is generated, the software program testing 
begins. Different testing methodologies are available to 
unravel the bugs that were committed during the previous 
phases. Different testing tools and methodologies are 
already available. Some companies build their own testing 
tools that are tailor made for their own development 
operations.

6. Maintenance

The software will definitely undergo change once it is 
delivered to the customer. There can be many reasons for 
this change to occur. Change could happen because of some 
unexpected input values into the system. In addition, the 
changes in the system could directly affect the software 
operations. The software should be developed to accommodate 
changes that could happen during the post implementation 
period.

there are some models depending apon the approach followed
to achive the target.
1. water-fall model
2. modified water-fall moedel
3.v model

1. water-fall model:
     in this SDLC model the succesive step is taken only 
after completion of the previous one. like first DESIGN is 
donefollowed by DEVLOPMENT , thereafter TESTING etc.
  The flow diagram if drawn,it can be said that IT IS LIKE 
A WATER FALL. so it called as WATER-FALL model

2. modified water-fall model:
  it is almost similar to the water-fall model but the only 
diffrnce is- the succesive step starts once the ongoin step 
is finished a bit. the following step starts..once the 
currnt step is partly completed.
 when the flow daigram is drownit overlaps the stages.

3. v model:
 as the name itslef suggests both design n devlopment steps 
start together...n at the end they meet the result. so it 
forms V shape.and called as v model.

- regards
 lalitkumar

SDLC models are
1.Linear Sequential model or Waterfall model
2.Prototype model
3.Spiral model
4.V model--same like waterfall model 

Above models are used in some cases like 
1.when small chnages are required to be done then waterfall 
model is useful
2.When project manager can give immediate feedback then 
prototype model is fine
3.when there are periodical increments then spiral model is 
correct