difference between transcriptional activator and general transcription factors?
Activators turn genes on - they help or promote RNA transcription of the gene.  Other Transcription factors may turn genes off (prevent or reduce RNA transcription).  Some transcription factors may promote or inhibit RNA transcription depending on their environment/context. 

Transcription factors is such a generic term that anything can happen in practice.  They are generically supposed to interact with the polymerase, but if some were found to interact by attracting other transcription factors that would not be a surprise.  

I think 20+% of human genes are transcription factors (that is just a guess we haven't been able to characterize more than a few of them.