What is Queueable Apex and how does it differ from @future?

Queueable Apex (implementing Queueable interface) runs asynchronously. Advantages over @future:

1. Accepts non-primitive parameters (sObjects, custom types, collections).
2. Returns a job ID for monitoring via AsyncApexJob.
3. Can be chained — enqueue another Queueable within execute() (max chain depth of 5 in dev/sandbox, unlimited in production, but limited to avoid infinite chains).
4. Can make callouts if Database.AllowsCallouts is implemented.
5. Can be called from Batch Apex. Usage: System.enqueueJob(new MyQueueable()); Testing: wrap in Test.startTest() / Test.stopTest() to force synchronous execution. Ideal for jobs that need object parameters or chaining patterns.