Crossovers are a huge part of modern comics. It's not uncommon to see Batman pop into Metropolis or for Spider-Man to swing by the X-Mansion from time to time and it's easy to lose count of how many times all of a company's characters have come together to stop a world ending threat or fight each other.
Less common is the intercompany crossover, an event where characters from completely different franchises meet. However, it has happened. Spider-Man has met Batman, the Justice League and the Avengers have teamed up and there was even an entire line of Amalgam Comics where Marvel and DC characters were merged into new heroes.
Then there are the times when things get weird. Sometimes superheroes will cross over with completely non-comic related franchises like movies, TV shows or even celebrities. As is inevitably the case when you've got two or more completely unrelated characters thrown together, these stories will often go to bizarre lengths to piece together an at least semi-coherent plot.
Other times they're just a transparent attempt to sell a product...