Purchase Options or Launch Types

Multiple customer instances run on same hardware
Different sizes of instances can run on same hardware
Per second billing while running.
Associated resources like storage might be charged even when instance is not running
No discounts. Can be interrupted as Reserved instances have greater priority

Long term consistent usage
Commitment made to AWS for long term consumption of EC2
If reservation is locked to AZ, it reserves capacity. If reservation is locked to a region that does not reserve capacity.
If an instance larger that reserved instance is launched, you get partial benefit
Commitment can be 1 year or 3 years
Payments can be no upfront. partial upfront or full upfront
- Scheduled Reserved instances, reserve capacity during certain times.
- Need to reserve minimum 1200 Hours per year for a period of 1 year minimum
Capacity reservations reserve capacity but do not give an discounts.
There is no one or 3 year commitment required
Savings plan are hourly spend commitment for one or 3 years
After committed number of hours, normal on demand rate is applied

Entire host is dedicated to you
No instance charge but per host charge
Typically used when using software having licensing based on sockets or cores
Host affinity links instances to host. So even when you stop & start the instance, its done on same instance.