ICANN explained

What Does ICANN Do?

To reach another person on the Internet you must type an address into your computer - a name or a number. That address must be unique, so computers know where to find each other. ICANN coordinates these unique identifiers across the world. Without that coordination, we would not have one global Internet.

In more technical terms, the Internet Corporation for Assigned Names and Numbers (ICANN) helps coordinate the Internet Assigned Numbers Authority (IANA) functions. The IANA functions are key technical services, and they are critical to the continued operations of the Internet's underlying address book, the Domain Name System (DNS). 
The IANA functions include:

• the coordination of the assignment of technical protocol parameters including the management of the address and routing parameter area (ARPA) top-level domain.
• the administration of certain responsibilities associated with Internet DNS root zone management such as generic (gTLD) and country code (ccTLD) Top-Level Domains.
• the allocation of Internet numbering resources.

How Does ICANN Work?

Besides providing technical operations of vital DNS resources, ICANN also defines policies for how the "names and numbers" of the Internet should run. The work moves forward in a style we describe as the "bottom-up, consensus-driven, multi-stakeholder model:"

Bottom up: 
At ICANN, rather than the Board of Directors solely declaring what topics ICANN will address, members of sub-groups in ICANN can raise issues at the grassroots level. Then, if the issue is worth addressing and falls within ICANN's remit, it can rise through various Advisory Committees and Supporting Organizations until eventually policy recommendations are passed to the Board for a vote.

Consensus-driven:
Through its Bylaws, processes, and international meetings, ICANN provides the arena where all advocates can discuss Internet policy issues. Almost anyone can join most of ICANN's volunteer Working Groups, assuring broad representation of the world's perspectives. Hearing all points of view, searching for mutual interests, and working toward consensus take time, but the process resists capture by any single interest– an important consideration when managing a resource as vital as the global Internet.

Multistakeholder model:
ICANN's inclusive approach treats the public sector, the private sector, and technical experts as peers. In the ICANN community, you'll find registries, registrars, Internet Service Providers (ISPs), intellectual property advocates, commercial and business interests, non-commercial and non-profit interests, representation from more than 100 governments, and a global array of individual Internet users. All points of view receive consideration on their own merits. ICANN's fundamental belief is that all users of the Internet deserve a say in how it is run.

Read more about ICANN here