To find a fault is easy; to do better may be difficult. -- (Arthur) Clive (Howard) Bell (1881-1964)

An art critic and member of the influential Bloomsbury Group, Clive Bell married Virginia Woolf's elder sister, Vanessa (herself an important British artist of the 20th Century). Perhaps marrying an artist (whilst himself being a critic) gave Clive the insight required to point out the rather obvious fact outlined above.
 

The difficulty with Development Methods

When one places even a small number of intelligent minds together, with the purpose of designing the 'best way' to do something, one is likely to end up with as many 'best ways' as there are people in the room (and possibly many more)!

However (at the risk of over-simplifying) there are two essential varieties of IT development methodologies: the first springs from the desire to achieve a 'correct' system that meets the original user requirements; the second aims for a 'good enough' solution (recognising the 80:20 rule and the likelihood that in fact requirements change over time).

The 'Correct' System

The desire to deliver 'correct' systems gave birth to software engineering (much in the same way as the desire to build bridges that did not collapse after a few months gave birth to civil engineering).

The so-called 'waterfall' methodology (of planning, analysis, design, construction & implementation) is essentially a serial method, where the completion of one stage is followed by another. It is also non-iterative, in that each stage produces a 'correct' result that does not then require re-working at a later stage.
 

The 'Good Enough" System

The power of waterfall methods is that they deliver reliable systems. However, in an increasingly competitive and fast moving business environment, relevance and timeliness have grown more and more important to developers.

So-called 'rapid application development' methods are based on the premise that it is better to be quickly there with a system that meets 80% of the business need so that the system delivers relevant business benefits sooner and does not become quickly overtaken by events & out-dated.

Rapid methods are essentially iterative (in that the outputs of the various stages are often re-visited during later stages). They are also mainly parallel methods, in that different stages may well be worked on at the same time.

In the waterfall method, documentation 'stands alone' at each stage, whereas in in rapid methods, documentation tends to be re-used and 'evolve' during the project.