Welcome to FT!

Differences among fare classes for a given cabin include:

1. Obviously, the fare.
2. Terms and conditions: advance purchase, refundability.
3. Frequent flyer credit earned.
4. Upgradeability: free if seats available, inexpensive, expensive, not allowed.

There is little or no standardization among airlines in these respects, except for two things:

1. A few codes, such as Y for the most expensive economy fare with the fewest restrictions.
2. When a given fare is sold out, all lower fares are also sold out. This is called "fare bucket nesting."

If people post that a given fare class is desirable, that generally means it earns more frequent flyer credit than others at a reasonable price. If posters try to avoid one, that's usually because it earns little or no such credit.

Many mileage runners are concerned with earning FF credit, especially to reach status levels in an airline's program. The details of how fare classes relate to that vary widely from airline to airline and can be complicated. It is impossible to recap them all here. Things get even more complicated when you factor in issues of earning credit in one program while flying with an alliance member or other partner airline. Check the forum(s) of the airline(s) you're interested in. The information may already be in a "sticky" near the top of the forum. If not, try searching. This information is also usually on the airline's Web site, but can be hard to interpret there.

Booking a specific fare class on an airline's Web site, or on a general travel site such as Expedia or Orbitz, can be a challenge. Many of them let you choose between, say, "refundable economy" and "non-refundable economy." Most let you find out what fare class a ticket will be in but don't let you do much about it if you don't like what you see. You must often speak with an agent to find out how much more expensive a more desirable fare class would be and, if the difference is small enough, to rebook a flight into it.