A Backspread can also be called a Ratio Spread. Backspreads are usually referred to this compilation when the strategy results in a net credit.
A Call Backspread is made up of a short ITM call and long two OTM call options.
The Max Loss is limited to the difference between the two strikes plus the net premium (which should be a credit).
The Max Gain is unlimited on the upside and limited on the downside.
When to use: When you are bullish on volatility and bullish on market price. Note though, that you profit when prices fall, although the gains are greater if the market rallies.
A Backspread looks a lot like a Long Straddle except the payoff flattens out on the downside. The other key difference is that Backspreads are usually done at a credit. That is, the net difference for both legs means that you receive money into your account up front instead of paying (debit) for the spread.
Even though the payoff looks like a "long" type position, it is often referred to as a "short" strategy. Generally it is like this: if you receive money for the position up front it is called a "Short" position and when you pay for a position it is called being "Long".