Real Time push APIs like Forwarding, Selective Forwarding are time sensitive APIs i.e. if your server did not respond within 10 seconds it will play the IVR and drop the connection of the caller. Please ensure your server responds in timely fashion always.
Queue based APIs like Billing or Call-To-URL keeps a queue of the data and sends the data one by one and are more fault tolerant but lags in response time.
For pull based APIs you are allowed 200 requests per day from a single userid and 500 requests per day from a particular IP address. Also if your IP fails to authenticate for more than 5 times in a day than it will be banned for next 24 hours.
API Key can be requested from here. API key is password independent i.e. you can change your password as many times as you want and it will not affect the API key.