Risk, Threat and Vulnerabilities are the technical terms that are frequently used interchangeably. Although they are related to each other, they have different meanings.

Consider a  situation like you are living in a house near forest. If you are keeping the door open then a Tiger or any wild animal can attack you and may kill you.

Here keeping the door open is the Vulnerability, Tiger/wild animal may attack you is the possible Threat and the probability of Tiger/wild animal coming through that door and attacking you is the Risk.

So,

Vulnerability refers to the weakness of a resource/asset that can be exploited by the attackers or threats.

Threat can be anything that exploit the vulnerability intentionally or unintentionally and can cause damage to resources/assets.There are natural threats like Earth quakes, intentional threats including malware,spyware and unintentional threats like an employee entering the wrong information.

Risk can be reffered as the potential loss or damage to resources/assets caused when a threat exploits a vulnerability.

RISK = THREAT * VULNERABILITY