Accelerator effect- when an increase in demand for consumer goods causes an increases in demand for capital goods (machines).
Example- If demand for Pizza goes up, then the demand for ovens would go up.
Multiplier effect- When you buy a TV you create jobs. How?
The person making the TV now has a job that would not otherwise be there had you not bought a TV. That person now has an income so he goes and buys a new sofa, therefore the person making sofas now has a job and so it carries on. In simple words spending encourages further rounds of spending or one person's spending is another's income.