NiceHash is a crypto-mining marketplace. The company is based in Slovenia, and it's currently the biggest marketplace in the cryptocurrency field. It is based on the concept of a sharing economy by connecting sellers and buyers of computing power from all over the world.
As a seller (Miner) you can use Nicehash either via its own GUI software which takes care a lot of the steep learning curve in the mining business such as algorithm switching profitability. In the "easy mode," the GUI takes care of all the details, and mostly auto sells user's computing power for highest possible profit on the marketplace. The dashboard provides sellers with additional value and data insights for better day-to-day mining.
Buyers are provided with access to massive hashing power on all significant hashing algorithms. This enables buyers to use hashing power to mine any coins; the established ones or the new coins on the rise. The unique pay-as-you-go purchase model allows buyers to be completely flexible and bid on the hashing power in real-time without long-term contracts.
If you are new to mining, make sure you read What is a mining pool