There is a limit to which the risks can be diversified and after 15 -20 securities. The risk becomes non–diversifiable.
Those types of risks which can be diversified is known as non -systematic risks.
These risks are specific to the firm or relevant to the industry.
Systematic risks on the other hand cover a broad range of industries. This phenomena could be rise in price of crude oil , Euro Debt crisis the hallow effect of these events have an impact on the securities of a wide range of industries.