Becoming certified is a process whereby you are trained in the safe use
of scuba gear & the procedures for scuba diving safely.

There is no government requirement.

Jim Wyatt already responded with part of the answer.  I'll add a bit to his
comments.

There are some fundamental things that you have to know to dive safely.
There's nothing particularly difficult about it, but it's essential stuff,
the kind of things that not knowing may result in no longer living.  The
most obvious thing you need to know is how to put the equipment together,
how to use it underwater and how to deal with some basic problems, a flooded
mask, putting a regulator back in your mouth and clearing it, and sharing
air with a diver when one of you has either run out of air (unforgivable,
but it happens), or has had a malfunction.  The less obvious things are the
effects of breathing air under pressure.  Included in this are the risk of a
gas expansion injury (gas expands as you surface, even when it's in your
lungs) and how to avoid it and how to deal with the issue of excess nitrogen
that is absorbed by your body during a dive.  Diving without knowing both
the obvious and the less obvious risks and risk control methods can get
seriously dead, seriously quickly.

The certification process includes classroom study, confined water practice,
usually in a pool, and open water diving, to demonstrate competence and get
a little experience actually doing what you learned about.

There's no government requirement for certification in the United States.
There is in some other countries.  Here, regulation is done by the industry
itself.  Most dive shops will not sell scuba equipment, will not service
scuba equipment and will not fill scuba tanks for somebody that is not
certified, i.e. does not have a card attesting to at least basic knowledge
about scuba diving.  You could find a way around industry standards, but
probably would be unwise to do so.

Lee

Just to add to the other great comments, there are a number of certifying
"agencies" throughout the world.

The four most common (pretty much in order) in the US are PADI (Professional
Association of Diving Instructors), NAUI (National Association of Underwater
Instructors), SSI (SCUBA Schools International), and the YMCA.

These are companies who have sets of standards regarding instruction and
certification.  An instructor gets certified as an instructor for one or
more of the agencies, and teaches classes for that agency, usually through
local dive shops.

Students sign up for the classes, complete the required cirricula, and get
sent certification cards in the mail from their respective agency.  I was
certified through SSI, whereas a few of my buddies were certified by NAUI or
PADI.  The class contents and requirements are virtually identical... there
is occasionally quite a debate over which agency is better, but a "C card"
from any agency will allow you the exact same stuff - air fills and dive
boat entrance the world over.

Classes are of varying lengths... mine was 6 6-hour sessions, one every
Sunday for 1.5 months.  Each was 3 hours classroom and 3 hours pool,
followed by two days of diving at a local lake with the instructor.

As Lee Bell said, it's not at all difficult, but there are some fairly
simple things you absolutely need to know.

Jon

I;m a newbie myself, taking a scuba class at UNC (f.cking Duke...)
Just from what I've learned in the first few weeks, I can tell you I am
really glad I'm learning diving from a diver who's been doing it for years.
Scuba isn't as easy as it looks, and if you don't know what you're doing,
you're gonna waste a lot of time "learning from experience", presuming you
live of course.
Aside from that, it's fun learning diving in a pool full of nice looking
college girls in bathing suits.

Some jurisdictions view actors and stage hands in SCUBA gear as light
weight commercial divers and their labour laws will require these
people to be trained and certified as such rather than as sports
divers. An Occupational SCUBA certificate in British Columbia is a
four to six week course and includes the 70 hours of bottom time a
trainee needs to meet the standards for certification.

Other provinces in Canada either have similar requirements or are
moving towards adopting them. Wouldn't surprise me if the American
OSHA and the British and Australian equivalents don't also have
something to say about diving standards on movie sets as well.

And yes, movie sets do get visits from local health and safety
inspectors. A acquaintance of mine almost got a set shut down when the
inspector asked to see his diving credentials and found he didn't have
his medical clearance with him.

Most governments in North America still don't care if a citizen goes
out and buys all the gear, a compressor, and goes out sport diving on
his own time without training or certification, beyond wishing they
could present his widow with a bill for the body recovery.

Sport diving certification started as an attempt by the sport industry
to regulate itself before the government did it for them, and has
expanded to limit the effectiveness of lawsuits against the industry.
It just means that a dive shop won't sell you air or life support
equipment like tanks or regulators UNLESS you can produce a card
showing you've been trained to use it. ALRIGHT YOU GUYS, QUIT
LAUGHING!