PurgeUsersOnline - This is a task that is used by the Users Online module and will remove users after a specific period of inactivity. The overall performance impact by this one should be minimal
PurgeSiteLog - This is a task that is used to purge the site log, if you are using the site log this is a very important task that prevents the log from getting too large. The performance impact is moderate, depending on the frequency of run and the number of records that it must remove
PurgeScheduleHistory - This is a task that is used to purge the history items from the scheduler and is of minimal impact
PurgeLogBuffer - This is a task that is used to purge the buffer of event log entries. THis is another minimal impacting entry as it simply takes the entries in the buffer and commits them to disk
SendLogNotifications - This is the task that sends log notifications to the admin, this is minimal, but can be a bit tasking
SearchEngineScheduler - This task is the one that updates the search indexes on your site. If you have a large site with many modules that support ISearchable this can be a VERY taxing operation on your system. if you only have limited updates, I would recommend running once every 12-24 hours to prevent server overload
InstallResources - This task is one that would automatically install resources, this is one that I would NOT schedule, and it can cause delays as it could force application restarts on module installation
SynchronizeFileSystem - This task is an automated file system sync, if you have a large number of files it can be a bit taxing on the server. If you do not FTP upload files, you might consider disabling this task.
PurgeCache - This is the one that purges cache entires after the specified time, the overall impact is minimal.
I will post a blog this afternoon that goes into a bit more detail on these entries