In pure technical terms, they’re quite different. I hate to sound glib, but an adapter adapts, a charger charges.

A USB power bank is also an adapter… it’s taking the power of a bank of Li-ion cells (usually), varying from 3.0VDC to 4.3VDC, and “adapting” that to deliver the +5VDC that USB-powered devices need.

A charger is a device that charges a battery, such as my trusty old Watson dual-charger here. You clip the battery into one of the plates on top and this device manages the charging of the battery, based on the battery type. If you look closely, you’ll see that the Watson has a USB jack on the side and can also act as a power adapter for any USB-charged device.

I have a small charger for my phone’s removable batteries in my kitchen, so I can always keep a spare battery fully charged. If you note the USB port in the side of this unit, that tells you that you can also use this as a power adapter.

Simply put, any device that’s going to run from low voltage DC current from high voltage AC plug power is going to need what we in electronics call a power supply. That’s simply a power conversion device, which will take one kind of power and convert in to the kind of power that you need. That’s pretty much all that a power adapter ever does.

The charging circuit is an extra piece. It has to have a charging management circuit that does the right thing for charging a battery, switching off once the battery’s done, etc. For Li-ion batteries, this can be done with a small microcontroller and some voltage sensors and power electronics, or it can be done with a dedicated battery charging chip.

For any device that charges from USB, the charger for that device is IN that device. So unless you have a removable battery, the only charger you will ever use for that phone is built-in to the phone. It’s actually part of a larger subcircuit in that phone, the power management system. If you didn’t have a power management system, your phone would probably last 30 minutes on a full battery. The power manager works constantly to decide which parts of the phone need power, which can be shut down or scaled down based on user demand. And when on external power, such as the output of a USB power adapter of some kind, the power manager will decide how much of that power is needed to run the phone’s operations, and if anything’s left over, it’ll get to charging your battery.

And yes, everyone calls those USB power adapters “chargers”. It’s the thing you grab when you want to charge your phone, and most people are not electronics engineers with decades of experience. So it’s a natural name… it’s technically wrong, but fairly harmless. But there is one profound observation possible in knowing this: your phone is entirely in charge of its own charging. So as long as you’re feeding it a regulated +5Vdc with enough current, the battery will correctly charge, without any possibility of damage. So there is no advantage in using only power adapter dongles made by your phone’s manufacturer.

And in fact, many phones ship with power adapters that do not charge the phone as fast as it could be charged. You may be better off with a third party adapter.