To clarify, consider an example:
- You own, say, an ice cream business.
- You buy a premises for $100,000.
- Two years later, you sell the same premises for $150,000 -- it's not that you've done anything to improve it at all, it's just it's the way real estate works.
Then you have made capital gains of $50,000. The thing that distinguishes from normal profits is that it's not the result of normal profit-making activities -- in this case, the increase in the value of the premises has nothing to do with selling ice cream. It's just a bonus.
Capital gains is also applied to the business itself, on what is called goodwill. The goodwill of a company is the difference between what the company's property is worth and how much it is sold for. For example, if a company owns $100,000 worth of stuff and owes nobody anything, but it gets sold for $200,000, then there is a goodwill of $100,000. Increases in goodwill are considered capital gains. This is also taxable, even though it could be argued that this was perhaps a direct result of the company's activities.
Some countries also have inflation-adjusted capital gains, in other words, you only pay capital gains on the sum over and above the increase in things' values due to inflation.