Proof of identity
Proof of your unfamiliar ethnicity or U.S. residency status
What counts as proof of identity for the IRS?
Passport (this can demonstrate both your identity and residency status)
A U.S. Division of State-issued visa
U.S. driver's license
Personal ID issued by U.S. Citizenship and Migration Services (USCIS)
U.S. military identification card
Unfamiliar driver's license
Unfamiliar military identification card
U.S. state identification card
Unfamiliar common birth endorsement
Unfamiliar clinical records (for dependents under six years old)
Public ID card with your name, address, date of birth, photograph, and lapse date
Unfamiliar citizen's registration card
Unfamiliar school records (for dependents who are students and under 18 years old)

Who can get an ITIN?
Any individual who needs to document a U.S. tax return can get an ITIN, regardless of their U.S. movement status. That includes both resident and non-resident immigrants.

You can apply for an ITIN in the event that you're:

The ward or spouse of a U.S. resident, resident outsider, or non-resident outsider visa holder;
A non-resident who needs to record a federal personal tax return;
A resident far off public working in the U.S.; or
Living outside the United States but working for a U.S. company.
Assuming that you're unsure whether you really want an ITIN, check with your employer.

IRS.gov has an abundance of useful information about the process for non-residents. They also have special resources for worldwide students, military members, and others.

What's the contrast between an ITIN and a SSN?
The Internal Revenue Service (IRS) issues ITINs, while the issuing office for the SSN is the Social Security Administration, or SSA.

An ITIN is a tax ID number. It serves no other purpose aside from federal tax reporting. This means that anybody who is required to record taxes in the U.S. will be qualified for an ITIN, regardless of their migration status.

An ITIN without help from anyone else does not qualify anybody to work in the United States.

A Social Security number is just issued for citizens and qualifying resident aliens, such as green card holders. This number keeps track of the years you've worked in the U.S. and your lifetime earnings as a registered agent.

Lawful residents who can legally work in the U.S. with SSNs can qualify for extra benefits such as:

Unemployment pay
Social Security disability
Access to federal loans and subsidies
Government medical care enlistment (medical advantages for seniors)
ITIN holders don't qualify for such Social Security benefits.