Can you explain the theory of evolution?

In simple terms, the theory of evolution by natural selection states that when

1. Animals produce slightly different offspring,

2. The differences are inherited by their offspring,

3. Animals produce slightly more offspring than can comfortably subsist with their niche,

4. The differences affect their chances of reproduction, so competition exists between different offspring, whether directly or indirectly,

Then traits that give these animals a reproductive advantage will increase their frequency in the population; more animals in subsequent generations will have these traits.

The result of this over many generations is that animals will change in appearance and behaviour, and ultimately, that populations in animals may divide into different branches, each evolving along their own path to form distinct new species.