Is it legal to buy a domain name similar to a real domain to generate ad revenue?

I'm wondering is it legal to buy a domain similar to a real domain with the intent of generating revenue from Ads?

For example, someone owns lenovo.ca which is separate from the official lenovo.com website. Being in Canada I sometimes go to lenovo.ca, and I'm assuming the owner gets money because of the ads on his webpage. He has not pictures of Lenovo or anything related to Lenovo's intellectual property on his website.

Is this legal? If so can I do the same thing if I can find a domain that is similar to a real domain?

I don't see how it can be, if the original one finds out, they can sue you and if they can prove you are stealingTheir name if you are not advertising and selling something.