A key is a string (without white spaces) to identify a user. It is something like this.
One key has basically two features. Total bought credits number and Total left credits number. Want a key with some free credits for testing? Contact us.
A task id is a number for identifying a captcha submission. It is returned when submitting captcha and it is used when sending correctness to server.
Vendor key can be used in your software if your are a software vendor. With this key, you can know how many captchas are submitted to our server by your software and so get your profit sharing from our partnership program. Want to get a vendor key and join our partnership program? Contact us.