append: append will place the values at the bottom of your search in the field values that are the same. for instance, if you have count in both the base search and append search, your count rows will be added to the bottom. i believe this acts as more of a full outer join when used with stats to combine rows together after the append
http://docs.splunk.com/Documentation/Splunk/6.5.1/SearchReference/Append
appendcols: this will add new columns to the base search instead of just appending it all to the bottom.
http://docs.splunk.com/Documentation/Splunk/6.5.1/SearchReference/Appendcols
join: this will also add new columns to the base search instead of at the bottom, however it is not a full outer join. join does take performance away and it is better to try to use other methods, such as stats, append, appendcols, etc.
http://docs.splunk.com/Documentation/Splunk/6.5.1/SearchReference/Join
