I'll try to give you a little comparison of spreads. Two types of basic futures spreads
Intra-market, Time spread, Calendar Spread, Settlement Spreads, Delivery Spread, etc.
Financial Futures: Professional stuff ranging from highly levered rate differentials to really complicated market making involving SWAP and other OTC markets. Used in various forms of arbitrage/hedging related to complex banking operations.
Index Futures: Arbitraged against funding rates (LIBOR, etc) illiquid, no vol
Agricultural Futures: Low vol that can blow out, can trade the whole curve e.g. fly's, seasonal stuff is common, tons of jargon and very deep. (crop spreads, convenience yield, full carry)
Metals Futures: Don't know, probably specialized knowledge required.
Energy Futures: Extremely serious due to the size of the actors involved. Retail is at a major informational disadvantage. The pro's have insane amounts of money, tech, and resources. Complexity ranges from simple to totally professional. Varied.
Inter-market, Cross-Hedge, Correlation Trade,
Futures Baskets, Relative Value, and Relative Performance
Financial Futures: Used by rates traders to control risk and speculate (NoB, BoB). Many different types of trades are possible here. Huge amounts of speculation going on. This is one of the most influential markets of all!
Index Futures: Used to trade equities with very high leverage, and to hedge international equities risk. Huge opportunity for smart traders to spread global indices!
Agricultural Futures: Different ways to play vol and take speculative positions. So varied can't really offer you much, need to study it to learn anything.
Metals Futures: Speculate on metals ratios in the front month. Used to hedge other market risk.
Energy Futures: Traders use these as a way to increase or decrease volatility. Hugely speculative.
The types can be combined and even more complexity can be added in. Traders trade all of these and more (for good reason). Most people don't have the necessary training, experience, or knowledge to trade more than a small part of this stuff.
I started a discussion on ET about index spreads here.
https://www.elitetrader.com/et/threads/index-spread-traders.335197/
Good luck in your trading!