I just installed 4.7.25. All messages said success. However, when I enter CiviCRM in my Joomla site, I see several warnings like No Cron running, etc. When I click on "View alerts...", the screen empties. Did a file/script get missed? Should I reinstall? Thanks for your help.
Welcome to the world of CiviCRM! A few things to note:
Joomla 3.8 changed the location of some files, breaking CiviCRM on Joomla. Some of these problems were fixed in CiviCRM 4.7.27, which was released yesterday. The other bugs were fixed, but not in time to make the testing phase for Civi 4.7.27. They'll be in Civi 4.7.28, releasing December 6, 2017. If you feel comfortable applying patches manually, see CRM-21203 and CRM-21208. Otherwise, you need to wait for the new release.
Like Joomla, CiviCRM doesn't set up cron on its own - you must set it up yourself. However, scheduled jobs tend to be more important in CiviCRM (e.g. you can't send out a mass mailing without cron, or auto-update membership statuses). Instructions to set up cron vary widely depending on your host - if you have direct access to cron, you can use the instructions on this page: https://docs.civicrm.org/sysadmin/en/latest/setup/jobs/. If you use CPanel, WHM, AlternC, etc., you can set up cron through the control panel.
Note that the cron instructions are a bit messy, and have been rewritten, but the rewrite hasn't been fully edited yet.