Earning Bitcoins, or any other cryptocurrency, can be done in multiple ways. In this article I have listed the most common ones.
Of all the ways to earn Bitcoin, mining is oldest one. Your computer – or your network of computers (a.k.a. mining pool) – does all kinds of calculations to solve mathematical problems. The computer or mining pool that generates the answer to such questions, will ‘win’ the bitcoin. Most cryptocurrencies, including Bitcoin, are setup in such a way that the amount of available bitcoins is finite.
Work & services
You can setup an online business and offer your services in return for payments in Bitcoin or another cryptocurrency. In 2010 a pizza delivery guy sold 2 pizzas for 10.000 Bitcoin, which would now be valued at 99 million dollars.
You can always buy cryptocurrencies at a certain price, and sell them later with a profit. This can be done against traditional currencies (US dollars and Euros), or against other cryptocurrencies (which is by far the most popular way to do it). Many insiders advice to only do this with the Top 50 currencies, which are considered more stable and less likely to be scam. However, the value of a cryptocurrency is highly volatile. Profits are there for the taking, but losses happen just as often.