many different "standards".

You mentioned threads, so I'm going to assume threaded rather than threadless.

With threaded, the majority are 1", but there are some other ones out there (French, 1 1/8", etc)

With 1", you need to know if it's JIS or ISO. If ISO, it can be British or Italian threaded.

Then you have to consider stack height.

So, to make this [maybe] easier, tell us what you can read on the headset you have, and what brand/age of bike you own.