しごと　is job or work and it is often used in professional work. For example , わたしのしごとは教師です。(My job is teacher.) 明日は仕事があります(I need to go to work tomorrow) ようじ　is something to do and it is personal work and not sociable one. For example , ようじがあるので休ませて下さい Please give me a rest because I have something to do. It is often used to deny invitation from others in business occasion.