There are four main resources in Clash of Clans: Gold, Elixir, Dark Elixir (introduced later, at Town Hall level 7) and the premium currency, Gems. The former three are used to construct various buildings and upgrade them, and the two Elixirs are also used to train troops. Gems are very hard to come by, and those are needed to buy Shields for protection, completing buildings quickly, and most importantly, buying additional Builders in addition to the default two. The most annoying restriction can be the Builders, even early on.
While the gameplay is very good and engaging, it is also slow at the same time, with construction, research and training times heavily increasing around Town Hall level 4-5, where the amount of generated and looted resources will be much less in proportion to the amount of them needed for everything. This is where the profits come from: Gems can also be used to buy these resources directly, then to skip waiting times, then to boost resource generation and to buy Shields to protect these resources from being looted.