What are good, bad and invalid shares?

Good share means that your computer has resolved the mathematical task given by the network, the solution was correct and your computer was among the first ones to submit this correct solution back to the network. These shares bring profit.

Bad shares mean that your computer has found the correct solution, but was too late to submit it. These happen occasionally, do not bring profit, but at the same time do not influence your mining process.

Invalid shares are the incorrect solutions that your computer may submit. Those are usually a signal of wrong mining process setup or mischievous activity.