HMO = Health Maintenance Organization. Basically, there are different HMO's , and most health care providers (doctors, dentists, etc) will accept certain HMO plans. You as the patient have one HMO plan, and to get insurance coverage, you can only go to those providers who accept your HMO plan. I'm sure it's more complicated than that, but that's the basics.
401(k) = A type of retirement plan. This is a join effort between you and your employer. Usually, you choose how much you want to put into your 401(k) (a percentage of your paycheck), and your employer will have a matching ratio up to a certain percentage/amount. For example, a 3:1 up to 5% means your employer will put in $3 for every $1 you put in to the 401(k) up to 5% of your paycheck. Then, you put this money into stocks (but you have limited options). Usually if the 401(k) provider your company is working with is any good, these stocks will be stable long-term type stocks. It's said that 401(k)'s are really good for those starting young because it leverages the power of interest. You don't pay taxes on the money accruing in here.
Roth IRA = Another of type of financial account. The great thing about this is that you do no pay taxes on any of the money in here and you can invest it in anything you want. Any money you make on this investment is also non-taxable. The catch is that you can only put in $2,000 a year.
In the case of the Roth IRA and the 401(k) you take a penalty for withdrawing the money too early, usually any time before retirement.