What is the difference between the USB adapter or just a USB adapter? I was told I needed one for my new iPhone SE

When Apple says "USB" they mean USB-A. That's the one with the rectangular profile. The USB ports on new Macs are USB-C which are rounded. See the Related Article link at top to see a picture. Of course you only need the adapter if you have a newer computer.

can you clarify what you are trying to accomplish?

The term USB adapter all by itself is extremely generic.

If you are having trouble because the SE came with a USB C to lightning cable.. I think your best options are to purchase a USB C power supply or get a USB A to lightning cable. If you still have cables from previous iPhones, those will work too.

Here is the Apple USB C charger..

20W USB-C Power Adapter

or one from Anker, they work great

https://us.anker.com/pages/uai2020

Or get a different cable that works with your old chargers

Lightning to USB Cable 

or, again, Anker makes good options here too.. one example below

https://www.anker.com/products/variant/powerline-iii-lightning-cable/A8813021

These have different sizes for the sockets and plugs.

USB-C is a newer technology that permits faster phone charging.

The iPhone socket is called Lightning. The included cable has a Lightning plug and on the other end a USB-C plug. If you buy a USB-C power adapter of 18 watts or more, you can use the included USB-C to Lightning cable to enjoy faster charging. However, you can still use the older power adapters, which come in 5, 10, or 12 watts, and use the older cords included with previous iPhones. These are a USB-A adapter, and a USB-A to Lightning cable. The only difference is that this type of cable carries less power, so it will take longer to charge the phone.