Sterling Silver consists of 92.5% silver - this is the reason why Sterling Silver is popularly referred to as 925 Sterling Silver or just 925 Silver. Genuine sterling silver never contains nickel, as it causes allergic reactions in many skin types.
The reason silver needs to be combined with other metals is that it is very difficult to make great designs with just pure silver because it is very soft and malleable. A bit of hardness has to be introduced, by adding other metals such as copper. That’s why jewelers are capable of making the most intricate and complex designs with 925 Sterling Silver.