The short answer
Porting moves ownership of a number to a new carrier; forwarding leaves the original service active and sends calls elsewhere.
Step by step
Choose a port when you want the same number to become the active number on a new carrier.
Choose forwarding only when the old account will remain active and you need calls sent temporarily to another number.
Check whether forwarding includes texts, voicemail, international calls, or additional charges; many setups forward calls only.
For a port, keep the old line active and provide the destination carrier with the exact account number and transfer PIN.
After either change, test calls, texts, voicemail, authentication messages, and billing before relying on it.
Before you finish
Keep screenshots, receipts, chat transcripts, confirmation numbers, and the first bill after a change. Carrier terms and account eligibility can change, so verify the final price and conditions directly before submitting an order.