Here is a quick excerpt from the guide;

What is Patent Licensing?
Legally speaking, you have licensed your patent when you (the licensor) grant exploitation rights over your patent to a licensee (the person you are licensing it to.) Exploitation rights simply means the right to create, market, and/or sell something based on what that patent protects. A license of this nature is also a legal contract, and that contract is what will spell out in concrete terms precisely which exploitation rights are being granted. These include any performance obligations the licensor might demand of the licensee. This means that if any performance obligations are included in the contract (ie, You must produce X number of sales by the year X.), and they are not met, this could lead to the patent licensing being terminated in its entirety. In this context, a license is also revocable ie, cancellable if certain terms and conditions are not met. This is a common characteristic of legal contracts in general, with special ramifications for patent licenses. The only way to grant someone irrevocable exploitation rights, it should be added, is to assign them the patent. Assignments, however, are permanent. They entail the sale or outright transfer of the patent by the assignor to the assignee. An in-depth exploration of patent assignments is beyond the scope of this article, but just know that they are an option if irrevocable exploitation rights are something you seek.)

We hope that is helpful.