What constitutes a licence can vary by country and with the IP concerned.
In the UK and some other jurisdictions a patent licence can be created expressly or impliedly by a written agreement or verbal agreement. In contrast an assignment of ownership of a patent must be in writing and signed by both parties¹ similarly a licence to use a UK registered Trade Mark must be in writing.

In most countries licences (and assignments) should be registered.
Registration requirements will vary by country and by the type of IP involved. Failure to register may affect the rights of the licensee (or assignee) if there are subsequent inconsistent dealings by the owner. Failure to register may also affect the award of damages where infringement by third parties occurs.

Licenses can be granted in respect of most intellectual property rights e.g. :

• Patents / Patent Applications
• Copyright
• Unregistered Design Rights
• Registered Designs
• Registered Trade Marks

Licenses involving Trade Marks should usually be accompanied by an agreement to enable the owner to control the quality of the licensee's goods and there should be great care to avoid any trade mark licence (especially one limited in some way by territory or field of use) confusing customers.