If you start Solr with the default configuration, there will be only one instance running. That’s enough if you have only one website which you want to index but if you have several websites, you probably don’t want them to share an index as you then would get search results from both websites.
Luckily Solr already brings the so called multicore feature with it. It allows you to run multiple instances that use different configuration sets and indexes.
This little howto will show you how to install / configure Solr to make use of the multicore feature.