a SWOT analysis is done to see exactly what sort of environment a company is operating in.
SWOT of course stands for
Strengths and weaknesses are typically internal to a company - basically what a company does really well, and what it sucks at doing. (Maybe it's got a very unique product, however it has shakey relations with its suppliers).
Opportunities and threats are typically external to a company. Opportunities might be, for example, a lift on trade barriers for easier importation, but a threat may be that more companies can compete as a result of that trade barrier being absolved.
Your question is incomplete (2.What is the purpose of SWOT analysis? Why is it necessary to do a SWOT analysis before selecting a particul), as well the #2 sounds like you should be doing your own homework instead of asking Yahoo :) But if you complete the question I'll add more.