Earlier quoted context omitted.
What's the difference between a simple certificate and something higher-grade? What does the simple certificate lack that a higher grade certificate provides?
Extended verification certificates (EV; "actually verified") cause the browser bar to turn green. That will make people more likely to trust you.
Will a free StartSSL certificate trigger an 'untrusted source' warning from the browser?
Also, will a free certificate be adequate for encrypting authentication data in a web API?