This chart is by no means comprehensive enough to include all options in all brands, but is accurate enough to help people start to categorize the bikes easily and maybe even help eliminate choices to help hone in on the category that they should be riding in, or buying in.
Cross Country (XC) is where many riders live who are sometimes racing, but more likely just want a faster bike or lighter bike. The bikes are not as capable flat-out through rock gardens or steep descents, but can handle most trails very adeptly. If the route is smoother and less technical, these bikes really come into their own with their sheer speed and handling. Take a look at the Cannondale Scalpel as an example of this style of bike. Some companies extend the travel a bit, raise the bars and put some bigger tyres on their XC bikes to make them more appealing to Trail riders, affording the riders a faster climbing, handling & accelerating bike overall - like the Scalpel SE.
The Trail category of bikes is where a bulk of the mountain bikers in the world live, therefore this is where most of the bike companies sell a bulk of their bikes. These bikes are all about balance - balance in weight vs durability, balance in descending vs climbing priorities. We might join an event every once in a while but probably ride most of the time with a few close friends or groups at local trails. Some of us like to push the technical side of things with some jumps, some skinnies, rocky switchbacks, flow trails, but few of us are racing anything but ourselves or our buddies. It's more about fun, fitness, and friendly competition than anything else. The bikes reflect this in a few ways like geometry, overall weight, components, body positioning, and gearing. GT Sensor and Cannondale Habit are great examples of trail bikes.
Enduro racing has led the development of a relatively new category of bikes in the brief history of mountain biking. Popular riding routes have a substantial bit of climbing and pedaling, but overall offer an attractive amount of satisfying downhill to keep the soul happy, sometimes shuttles or chair lifts, but often not. This requires even more durability and capability with suspension, geometry and components than a trail bike, but without giving up on the pedaling efficiency and weight altogether. There is definitely a bias towards descending and big smacks on the wheels from landing big drops, but they still can climb and have big gear ranges to support it. The companies developing bikes in this space have delivered bikes that are super appealing to the adrenaline junkies and bikes like the Cannondale Jekyll and GT Force are perfect examples.
Downhill Racing and Downhill-specific bikes have evolved as well, even though they have been on the scene since the earliest days of mountain biking. The downhill bikes of today have fewer gears, all high-speed gears and almost nothing for climbing. The geometries are made to keep the rider low and far back behind that front wheel that is dropping low and hitting hard and fast. No dropper post, no full leg extensions, just sitting on the saddle for a brief rest before the next fury of pedaling and jumping. The dual-crown forks, gobs of travel, and very slack geometries reflect this very specific purpose.
In conclusion, there are more choices in brands and models, price points and specs, than ever before, but it's helpful to know that most bikes fall into easy-to-identify categories.
I get asked often "How do I choose what model to go with?" I say when choosing from bigger companies like Cannondale, GT, Specialized, Trek, and Scott, you get a team of suspension and materials developers behind you that really make the bike shine when the trail gets rough. Warranties, small parts availability and long term support are also better when going with a bigger brand. There might be some ideas, innovation, and cool stuff going on with a few of the boutique brands, but you can't go wrong with a bigger brand's popular model for longevity, performance, support and the ability to get parts when you inevitably need them.
