I have spent a great amount of time in bike shops. One of the questions that people always seemed to ask that everyone answers differently is "What kind of bike do I need?". Today I am going to map out each of the predominant styles of bikes and give this to the world so people stop buying hybrids for the local mountain bike trails or whatever the awful situation may be.
Triathlon/time trial bike:
This is a top fuel dragster. Really fast in a straight line and not good for much else. Only ride it on nice, smooth roads, don't try to haul anything (ride with a backpack), and don't expect it to be cheap - now or continued maintenance.
This is just like the average sedan. You can ride a road bike in grass, gravel, dirt, and all roads you can find. It really is just like a sedan. You won't take it on gnarly backcountry forest roads, through mud and sand, or do many sweet jumps with it before you have to visit the mechanic.
This is like a rally car. It looks like a car, everything looks the same . . . wait, no it doesn't. It has a little more clearance, Clarence. The tires are bigger. There are big mudflaps on this thing (canti or disc brakes - you know, it looks different). You know you want one. You know you are going to tear something up and get hurt. But to fly through the air while people are lined the course only to land a little crossed up and end your weekend when you taco your front wheel but the roars get louder from the fans - that's the fame you are looking for.
Subset of cross bike - holds bigger tires and people probably aren't lining the course cheering. You definitely don't pass the pit every lap. No one wants to come watch.
This thing looks like a mountain bike (more on that one to come) but its really the nasty, breaching the water, won't flush, butt nugget of the bike industry. They are like a crossover SUV - you know those awful SUVs built on a car chassis so they don't have enough power, good suspension, and can't haul anything but still get bad gas mileage and everyone made fun of you because you were stuck in wet grass once. That's how I feel about hybrids.
This is the bike everyone should buy instead of a hybrid. This is like a two-wheel drive Jeep Cherokee from 1995. They are just rugged enough to do whatever you might want to do on pavement, gravel, grass, and even a tiny bit of mud. They can haul stuff (usually have rack and fender mounts) and they are pretty cheap and last forever.
This is a Jeep Wrangler 4X4. Nothing says party like showing up on your mountain bike with a sixer of craft beer in your Camelbak, some big obnoxious light on the top (of your helmet) for your sweet night time wheelin' adventures. Cuz getting nasty in the dark is more fun than in the day time. Also, just like a true Jeep enthusiast mountain bikers are fanatics about tire pressure and "How big your tires is" and "How much lift" (suspension travel) does your rig have. Also - probably the only category of people who carry spare parts other than a flat repair kit with them on the ride.
This isn't really lumped into mountain bikes because they have other uses. They are like a semi truck. They usually aren't stupid fast and a ton of people (comparatively) aren't using them. The people that like them, love them. And let the big wheels keep on turnin'. If you don't get it, don't hate. Some people need 'em.