Mining for cryptocoins is analogous to mining for gold.

Little guys are mining with spades and shovels while the big guys are mining with excavating equipment. This is known as your "hashrate". We are all digging in the same spots looking for the same coins and the big guys simply have a greater chance of finding coins before we do.

When coins are found you solved a "block" and are rewarded with the coins you find. The more coins we find the less there are to find, so we have to dig deeper and longer to keep finding coins. This is known as the "difficulty" of the block, which increases with time.

In order for us little guys to stand a chance we band together in whats called "pools" and all dig together with our little shovels. If one of us finds coins (i.e. solves a "block") then we share the reward equally depending on how hard each individual shoveled (i.e "hashed") during that time.

The better your computer is the higher your "hashrate" is. Link multiple computers together and your hash-rate couples together. Link 100's of computers together and you now have a mining farm.

All of the crypto-currencies are like a precious metal. Think of bitcoin as gold, ethereum could be silver, Dash coin copper, etc. Depending on which one you dedicate your resources to determines which one you end up mining and obtaining in the end.

If I try to mine for gold but there is not much gold left to mine than the chances of finding any are SLIM with a shovel. Same with silver, even copper. But there are MANY metals and minerals that can be mined and all have variable prices and amounts available. Who knew back in the 60's gold's value would multiply 60x in less then 50 years?

Most cryptocurrencies are no different than national currencies other than the fact that there is a (typically) finite amount of currency to be generated. No more can be "printed" or generated after the limit has been reached. It is inflation proof and the value must go UP if it is mass adopted. Anybody, anywhere and anytime can generate a new cryptocurrency that MAY be mass adopted worldwide.