"Independent" refers to candidates who are on the ballot but are unaffiliated with the Democratic or Republican parties. 
"Other" refers to candidates from any number of third parties who are on the ballot. 

Notes

The CNN Polling Center displays data from polling organizations that use CNN-approved polling methodology as well as some that do not.  The asterisk symbol * indicates that the poll or polling organization does not meet CNN's methodological standards.  A CNN Poll of Polls is calculated using at least three polls that use CNN-approved methodology.  The Congressional Ballot polling data was provided by PollingReport.com.