In Bottom Up Methodology, we build the bottom-most layer

first and then ascend onto the actual application i.e. all

the smaller chunks of code or assisting functions are build

first and then the bigger function which uses these are built.

Bottom Up methodology is followed when we are definite about

the number, type and functionality of the Bottom most

functions(Assisting Functions) to be defined. Once all these

functions are defined, we build the bigger system above it.

Top down approach is followed when we are not very sure

about the number/type of functions that need to be

implemented, so the application is started to be implemented

and as and when smaller functions are needed, they are built.