What are Indices & How to Trade Them?

Index examples include
USA - Market we have the Dow 30 (made of 30 stocks) - very tight spreads
Germany - DAX 30 - this is an absolute beast to trade - at times it can swing wildly. I love the DAX as it can give you those regular runs..
France we have the CAC 40
UK we have the FTSE 100

If you're trading an index, make sure you understand the overall mix making the index; is it mainly financials, mining, retail, telecom - what is the overall mix?