Tax avoidance involves bending the tax rules to try to gain a tax advantage that was never intended. It usually involves contrived transactions that serve no real purpose other than to artificially reduce the amount of tax that someone has to pay. It is not the same as effective tax planning but is often promoted as such.
Tax avoidance does not pay, as most schemes do not work. People who use tax avoidance schemes can end up paying not only the tax they should have paid, but also interest and potentially a penalty. This is on top of the fees they paid to the person or business offering them the scheme. Everyone is legally responsible for their tax affairs and for paying the correct amount of tax even if they get someone else to do it for them.