Duck, my darling, you have the most esoteric questions on these forums!
This is not a case of a rose is a rose is a rose. A Roth IRA is very different from a Traditional IRA, and both IRAs are different from a 401(k) plan. It's like a duck is not a goose or something.
A 401(k) is generally an investment vehicle that a company makes available to its employees. [For employees in universities or research facilities, the plan is called the 403(b), based on the particular IRS code section]. In a 401(k), the employee contributes a percentage of his income, and the company may or may not match those contributions. The money is invested in a variety of places of the employee's choosing. For example, many plans work with the big players like Fidelity or Vanguard so that employees can invest in the funds managed by those firms. The company HR types will let you know the maximum amount you can withhold. This money is intended for your retirement although many people "loan" themselves their own future. If and when you separate from the company, the portion of your own contributions are yours to keep. The company's portion is also yours depending on vesting rules. You do not have to keep the 401(k) with the company when you leave. The smart advice is to transfer the whole thing into another account.
The Traditional IRA (not the Irish Republican Army, hee hee) is a personal retirement savings plan whose tax advantage comes in the front end as opposed to the back end for the Roth. Depending on your income, Traditional IRA contributions (up to a maximum set by the IRS) are tax deductible in the year in which the contributions are made. Any withdrawals (termed "distributions") later on will be taxed at your tax rate then, which hopefully is lower than during the years of your highest earning power.
The Roth IRA is a slightly different twist in that contributions (again up to maximums set by the IRS nazis) are not immediately deductible, but contributions are not taxed either.
I am not sure I understand your last fact pattern. Most people (and particularly poor CFIs and regional FOs) do not have much money to invest in anything, let alone invest in both Traditional and Roth. If you have that much, can I ask you for a loan? hee hee. The general rule is that the more money you put away, regardless of Traditional or Roth, the better off you will be come retirement.