The actual page rank value is somewhat along a logarithmic scale and is not a whole number. The toolbar shows an approximation of the actual page rank value in whole numbers. In that sense it is not very reliable or accurate.
There is no difference between them. Basically Google toolbar is a facility provide by Google search engine which have the following options available for webmasters to have check out the website page rank, similar pages and back ward links.
Another difference is that Toolbar pagerank doesn't reflect the actual pagerank of your website at that specific moment. Toolbar pageranks gets updated every few weeks/months, so like previous posters mentioned, it's not very reliable.