What is a system? Where exactly the testing activities begins in SDLC?

System: 

  The term system is originated from the Greek term “Systema” which means to “place
                  together”. It can be defined as a set of inter-related components having a clear boundary that
work together to achieve a common set of objectives.

Testing Activities in SDLC: 

In SDLC the testing activity takes place after the coding phase where the software is evaluated.
The given outputs are compared with the required outputs.  Thus a decision is made about the
validity of developed product. If any problems found they are removed and the new product
undergoes again this testing procedure.
One of the software testing principle say that “Start testing early” in the SDLC. It has been
observed that most of the errors introduced in the requirement or design phase. Defects identified
later in SDLC are expensive to fix than defects identified in early stage. So testing should start
early to the introduction of defects in the early phase. So start testing early in the software
development would solve the problem, as the earlier you find a bug, the cheaper it is to fix it.