Impact of tax:The burden of tax on the initial person
Incidence of tax:The final resting place of the tax burden.
The person on whom tax is initially imposed may either bear the whole burden or pass part or the whole burden to someone else. E.g. for manufactured goods, the impact of the tax is on the manufacturer and the manufacturer may pass the incidence of the tax to the consumer.
If the manufacturer only passes part of the burden to the consumer, then the incidence of the tax wil be partly on the manufacturer and partly on the consumer.