A little tutorial I wrote lately, hope it helps!
Application Development Methods
There are two methods for developing application code: top-down and bottom-up. Each method offers a different development perspective, and determining which is most appropriate is a case-by-case or application-by-application decision. It may be easier to develop a particular application one way but develop another application with a different method.
Essentially, the difference between the two methods is that the top-down method iterates development by starting with the big picture of the domain (the top) and works downward by decomposing the domain into assemblies, namespaces, and classes (the bottom). In contrast, a bottom-up method, which is also an iterative process, starts at the class level (the bottom) and works upward to the top, by composing classes, namespaces, and assemblies. Historically, object-oriented design and development theorists have advocated decomposing a complex domain into manageable units, which is a top-down method. However, many pragmatists have since recognized that there are times when starting with a vague abstraction is counterproductive and a more tangible approach (i.e., starting by developing classes) is preferred.
A top-down development method is one of two commonly used approaches to develop an application (the other is the bottom-up method).
An application is developed as an iterative decomposition of the domain. The process starts with a high-level abstraction (clouds) and works downward to implementation (code). Where A top-down method is used in developing domain or enterprise applications.
The top-down approach is commonly used because it follows a traditional object-oriented design, which advocates that complexity is best understood by starting with an abstraction and decomposing it into smaller units. However, although that approach usually works, there are times when working with an abstraction is problematic and it is more productive to start with the basics and work upward (bottom-up method).
The domain is viewed from the big picture, and application development is commenced by developing an architecture, then working through assemblies and namespaces to develop classes. The architecture is prepared before code is developed, and it may be tweaked as code development progresses.
The Standard: Top-Down Method
The standard acknowledges the use of the top-down development method to develop a solution by developing an application by decomposing a domain problem.
A bottom-up development method is one of two commonly used approaches to developing an application (the other is the top-down method).
An application is developed as an iterative composition of the domain.
A bottom-up method is used in developing domain or enterprise applications.
The bottom-up approach is used because it starts off more tangibly by developing small units of functionality (classes) and incrementally composes the complexity from class level to assembly level.
The domain is viewed from a detailed picture, and application development is commenced by developing classes and working upward to develop the structure (e.g., namespaces and assemblies). The architecture is prepared as part of code development; it evolves as a consequence of assembling the solution.
The Standard: Bottom-Up Method
The standard acknowledges the use of the bottom-up development method to develop
an application by composing a domain solution.