EVE New Citizens Q&A

It was part of the information provided with EVE Onlines API service.
The key was used along with API key a while back, but has since been discontinued.

I assume you are asking, because an outdated 3rd party application is asking you to provide it.
The userID is no longer used with the new API system. Instead you are supposed to use a keyID and a Verification Code (vCode), which can be found at https://support.eveonline.com/api/

For more information on API Keys, please see the wiki article or this detailed article on a 3rd party site

I hope this answers your question

There is no "User" ID. There's the IDs you get from your API key, which ISD Etetia nicely explained, and Character ID, which identifies your particular character. The latter can be useful for finding portraits of your character, coding websites that have to link to it, etc. Tau gave one way to access it, but I have a simpler one:

Right click on your portrait anywhere it appears on Eve Gate, and copy the URL of the image. It will be something like this (this one is mine):

https://image.eveonline.com/Character/1270580029_128.jpg

The number after "Character/" and before the "_128" (or another number there depending on image size) is your Character ID. In this case, "1270580029".