Spark plugs ignite the fuel in a gasoline engine. Worn plugs misfire, waste fuel and can trigger the check engine light. Diesel engines don’t use them.
It depends on the type. Basic copper plugs last about 20,000 to 30,000 miles (32,000 to 48,000 km). Iridium or platinum plugs often last 60,000 to 100,000 miles (100,000 to 160,000 km) or more. Your manual lists the interval.
A flashing check engine light with a shaking engine means a serious misfire that can damage the exhaust. Reduce speed and get it checked soon.
No. Electric cars have no spark plugs, and neither do diesel engines, which use glow plugs for cold starts.
On some cars, yes. On others they’re hard to reach. If unsure, have a shop do it.
General guidance, not professional advice. Intervals are typical ranges and vary by car, so check your owner’s manual and ask a qualified mechanic if you’re unsure.