What a SWOT analysis is
SWOT maps internal factors (Strengths, Weaknesses) against external ones (Opportunities, Threats). It's a fast way to structure a situation before deciding what to do.
Keeping internal and external straight
Strengths and weaknesses are about you; opportunities and threats are about the market. Mixing them is the most common SWOT mistake. The grid above keeps the four cleanly separated.
Turning a SWOT into action
A SWOT is only useful if it drives a decision: pair a strength with an opportunity, or shore up a weakness against a threat. End every SWOT with a "so what".