Scholar can also include citations by non-peer reviewed sources as Alexander mentioned. This can be for instance be a master thesis. Scholar is most likely faster since the results are not gone through by people. More information about differences can be found here: http://libguides.lib.msu.edu/pubmedvsgooglescholar
My impression is that google is very fast to detect new citations compared to other sites tracking bibliographical stats. Citation data appears on google within days of a paper gets available online. There is a much longer lag time between publication date and the new citation to show up on scopus or ISI. Scopus on the other hand seems to be better on picking up citations in supplementary material compared to scholar.
Scholar can also include citations by non-peer reviewed sources as Alexander mentioned. This can be for instance be a master thesis. Scholar is most likely faster since the results are not gone through by people. More information about differences can be found here: http://libguides.lib.msu.edu/pubmedvsgooglescholar
I think that ISI citation is more trusted in the way that it only takes into consideration the officially ranked journals, contrary to google, sometimes it counts on non indexed journals.
One key difference is that self citations are not distinguished by google scholar but ISI gives you total as well as non self citations records separately
The results show that ResearchGate has indexed impressively many citations for a single website and has become a major source of academic papers, perhaps even starting to challenge Google Scholar in this regard