What are the differences between a PPO and HSA health insurance plan
and what are the major considerations that need to be taken into
account before making a choice between the two?

A Health Saving Account (HSA) is not insurance.  It is an account you
can set up where you put money in pre-tax (the money you put in is tax
deductible) and then you must use the money during the same year on
any medical expenses.  If you fail to use all the money that you put
in, then you lose the remainder.

A PPO is health insurance that typically has "in network" health care
providers that you can use fairly cheaply (insurance pays most of the
bill)... or you can use a "not in network" provider at a greater cost
(insurance pays little of it).

You can have a PPO and a HSA at the same time.  

Starting in 2005, there was a new type of insurance plan called a High
Deductible Health Plan (HDHP).  This plan (as the name suggests) has a
high deductible that you must meet before the plan starts paying most
of your medical bills.  At the same time, the plan deposits money
(Aetna deposits $125 per month) into a Health Savings Account that is
attached to the HDHP.  This account is similar to the HSA mentioned
above, but the money does not go away at the end of the year... also,
both the insurance company and the individual can place money into
this account.  These plans are very good for a fairly healthy
individual since the money will continue to accumulate over time.

I know that the Federal Govt offers HDHPs, but I'm not sure how many
businesses do at this time.

In fact, HSAs generally carry over from year to year, and /must/ be
accompanied by a HDHP.  There are other similar types of plans,
including Medical Savings Accounts (MSAs) and Flexible Spending
Arrangements (FSAs), each of which has slightly different rules, and
some of which are "use-it-or-lose-it" --  that is, unspent money is
lost at the end of the year.  To learn definitively and in detail
about these types of plans (although it doesn't discuss PPOs, the
other part of your question), consult IRS Publication 969, "Health
Savings Accounts and Other Tax-Favored Health Plans":
http://www.irs.gov/pub/irs-pdf/p969.pdf

Also, you might want to discuss this topic with your health care
provider, financial professional, and/or insurance broker.  Some
likely considerations are the general state of your health (for
example, any known, recurring, health expenses), your overall
financial situation and risk tolerance, and the degree to which the
various plans and companies allow you to use your current or preferred
doctors.  Many (all?) health insurance companies can provide you with
price quotes and booklets explaining their various products.