There are two classes of frequent flier miles: status-qualifying miles and award miles.
Status-qualifying miles are miles that you can only earn by flying with that particular airline. Those miles are used in calculating your "status" with the airline. Achieving a higher status generally comes with perks that make frequent travel easier and more enjoyable (such as being able to sit anywhere in the plane for free, extra checked bags, access to lounges and better chances at upgrades).
Award qualifying miles are the miles that people most commonly associate with frequent flier miles. These are the miles that can be redeemed for discounted flights in the future. There are several ways to earn this aside from flying with the airline (though flying with the airline gives you more award miles than not): credit card spend, promotions through airline partners, giveaways, etc.
There is also another metric that airlines collect to determine loyalty: the amount of money you spend on your tickets, or status-qualifying spend (or dollars). In the past, this information was only used to protect more frequent or high-paying customers from being inconvenienced during travel. Now, airlines use it as an extra requirement for achieving status levels.
Now, since award miles for airlines can be collected without flying with them, flyers with very little status-qualifying spend will usually be the first in line for involuntary removal (involuntary denied boarding, or IDB). However, IDBs have become less frequent ever since the United Express incident last year; airlines are trying to favor giving people vouchers first (voluntary denied boarding, or VDB) before bumping folks.
Status-qualifying miles are miles that you can only earn by flying with that particular airline. Those miles are used in calculating your "status" with the airline. Achieving a higher status generally comes with perks that make frequent travel easier and more enjoyable (such as being able to sit anywhere in the plane for free, extra checked bags, access to lounges and better chances at upgrades).
Award qualifying miles are the miles that people most commonly associate with frequent flier miles. These are the miles that can be redeemed for discounted flights in the future. There are several ways to earn this aside from flying with the airline (though flying with the airline gives you more award miles than not): credit card spend, promotions through airline partners, giveaways, etc.
There is also another metric that airlines collect to determine loyalty: the amount of money you spend on your tickets, or status-qualifying spend (or dollars). In the past, this information was only used to protect more frequent or high-paying customers from being inconvenienced during travel. Now, airlines use it as an extra requirement for achieving status levels.
Now, since award miles for airlines can be collected without flying with them, flyers with very little status-qualifying spend will usually be the first in line for involuntary removal (involuntary denied boarding, or IDB). However, IDBs have become less frequent ever since the United Express incident last year; airlines are trying to favor giving people vouchers first (voluntary denied boarding, or VDB) before bumping folks.