What are the functions of Helper T cells and Cytotoxic T cells?

Cytotoxic T (or CD8) cells are activated in your lymph nodes by dendritic cells. Once activated they are sent out to the site of infection and they bind to the cells that express the MHC class I that are presenting the foreign antigen. They will then release perforin (punch holes in the infected cell) and granzymes (induces apoptosis). The infected cells are now destroyed. Memory cells are made at this point for future protection and FasR & CTLA-4 is released from the helper T cells to disable the cytotoxic T cells.
There are two different kinds of helper T cells. Th1 and Th2. Th1 functions to empower macrophages so that they can destroy intravesicular pathogens (bacteria). This is done by two signals. Binding of CD40L from Th1 to CD40LR on the macrophage and by interferon gamma released from Th1.

Th2 cells function to activate B cells. Normally it is insufficient for an antigen alone to stimulate the B cell to activation. The Th2 cell when activated will bind to the B cell (CD40L and CD40LR) and release cytokines, primarily IL-4, IL5 and IL-10. This will now activate the B cell into proliferation so that it can send out antibodies against that pathogen.