Where can I find clear examples/explanations of options?

I have been trying to understand options before I get into buying or selling.

So far I know that Calls means I have the right to buy a stock, and Puts is that I have the right to sell a stock. But during my research I found conflicted things. Some say I can exercise my right to buy any time during the contract period while others said I can only exercise that right at the end of the contract.

The more I look for clear explanation the more confusing it gets. So I was hoping you could help me find the right place to understand this,