What is Hezbollah and what does Lebanon have to do with the Israel-Hamas war?

As the Israeli military and the militant group Hezbollah exchange fire over the border separating southern Lebanon and northern Israel, there are fears the raging war between Israel and Hamas could ignite a wider regional conflict. 

Below is a look at what's going on, the background to the long-simmering tension between Israel and Hezbollah, and what the risks are for the region and the world.