Generally the mortar was just a metal cylinder attached to a bipod mount. There was a projecting pin inside the base of the tube so that when the mortar bomb was slid down the tube a detonating charge was set off that flung the bomb into the air. The bomb itself was usually made of cast iron and varied somewhat in size and design. They were originally equipped with timed fuses but these were replaced quickly by impact fuses. Mortars themselves could be found in several, slightly varied shapes and sizes.
Mortars were used to hurl their bombs high into the air at steep angles. The desired effect was that the bomb would fall from directly above, landing right on the enemy. This kind of weapon was made very useful in trench warfare, as you could attack your enemy without leaving the safety of your trench.
Mortars were used to take out enemy machine gun posts, suspected sniper posts or other designated features. Larger mortars were occasionally used to cut enemy barbed wire, generally in situations where field artillery could not be used.
Although the British managed to produce the most effective design, the Germans were the first to utilize their potential during WWI. German military observers of the Russo-Japanese war left with a renewed respect for the mortar bombs they'd seen deployed. They began stockpiling mortars in the few years prior to the war and by the time war had hit, they had around 150 mortars available to them. The German's had envisaged how useful mortars would be against the eastern fortresses of France and this was their initial motive for stockpiling them.
The French and British were caught completely off guard by the reinvention of the mortar. The French reacted initially by digging...