What is the difference among isolation transformers and step up or step down transformer?

Actually an isolation transformer may be physically the similar as a Step up/Step Down transformer. The main difference is in the way they are used.

Another difference is that in a normal transformer there will be capacitance among the 2 windings, between the windings and core, among the core and shell, etc.

These capacitances bring in high frequency noise from outside, which will be transmitted in the secondary circuit. So [in an isolation transformer] wires are connected among each component, (not direct contact, but with insulation present). This allows a leakage current and eliminates unnecessary capacitance.