I belong to American, United, Northwest, and Delta's mileage programs (mostly because I travel all the time for work). You generally earn 1 miles per mile flown. For business/first class, you may earn more. It normally takes 25,000 miles for a free ticket in the domestic U.S. - but you need to book free tickets far in advance, because the # per flight is severely limited. I normally use my for first class domestic tickets (45,000 per flight) because it's easier to get what I want. For international flights, normally around 40,000 from Oct - May, and then 60,000 for the rest of the year (in coach).
Sometimes, airlines will have specials for short haul (less than 750 mile) trips, and you can then buy these tickets for 15,000 miles. There is normally a $5 - $10 fee to book mileage tickets if you do it online. Also, if you book last minute, you may pay up to a $100 fee.
You can also use miles to upgrade your ticket. I did this for DS and BIL for their honeymoon to Hawaii - I think is was 15K each way per person to upgrade them to first class.
If you fly 25,000 or more miles a year, you can obtain status (e.g., Gold Status on American, Silver on Northwest, etc.). Status allows you to request upgrades to first class (which you may still need to pay for) and also gives you bonus miles.
I fly on American as much as possible because I have Executive Platinum status (100K miles flown per year). This gives me automatic upgrades on every flight, space permitting, and I get a minimum of 2 miles per every 1 miles flown. I also have an American AAdvantage Mastercard - fee of $50 or $75/year, and I earn 1 miles for every dollar spent.
DH and I use our miles for "splurges" - in Dec. 2004, we flew 1st class on Quantas to Australia - this was wonderful (esp. when you spend 17 hrs on that plane!). It was about 270,000 miles for both tickets.
Let me know if this helps at all, or if you have specific questions, just PM me.