Hybrids are a blend of different bike types, as the name suggests. Some are flat-bar road bikes, trekking bikes are closer to tourers, and multi-use hybrids have more mountain bike in their DNA.
Hybrids are a blend of different bike types, as the name suggests. Some are flat-bar road bikes, trekking bikes are closer to tourers, and multi-use hybrids have more mountain bike in their DNA. More rugged than a road bike off road and quicker than a mountain bike on road, the multi-use hybrid isn't so much the best of both worlds as a sensible compromise between the two. If you'll only have one bike and will also ride off-road, it's a decent option.
A multi-use hybrid looks very much like a hardtail 29er. That's a mountain bike with a suspension fork but no rear suspension (hence, hardtail) and 29-inch wheels, which are the same diameter at the rim as the '700C' wheels of a hybrid. Look closer at the frame, however, and you'll see commuter-friendly frame-fittings that the mountain bike lacks: eyelets for a rear pannier rack and a full-length rear mudguard.
Some multi-use hybrids have a mudguard-ready suspension fork, with dropout eyelets and a drilled fork arch. It's possible, albeit fiddlier, to fit a full-length mudguard without these. You or your bike shop can attach the mudguard stays to the fork legs using P-clips (typically 28mm) and the top of the mudguard to the fork arch using cable ties. If the bike has a rigid fork, which is an advantage at cheaper price points, it will likely have the necessary eyelets.
A multi-use hybrid may have a shorter top tube than the equivalent cross-country mountain bike, to give a riding position that's more upright and relaxed rather than racy. The handlebar will be narrower, which is better for aerodynamics on road but not so good for control on extreme terrain.
Of course, a multi-use hybrid isn't meant for extreme terrain. The aluminium frame should be strong enough but the fork will be a short-travel unit with 60mm or so of travel, intended for easier dirt or gravel tracks rather than rocky singletrack. The suspension medium will likely be a coil spring. Hybrids costing closer to £1,000 may have an air fork, which offers lower weight, better performance, and fine-tuneability. Whether it's coil or air, the fork needs a lockout switch so you can stop it bobbing uselessly on smooth roads. Look for 'rebound damping' if you want better off-road control; it will stop the fork springing back like a pogo-stick.
The most difficult compromise for a multi-use hybrid is the tyres: traction and control on dirt requires soft, wide tyres with prominent tread lugs – the opposite of what you want for efficient rolling performance on road! Most multi-use hybrids will have tyres 35-45mm wide with a light tread in the centre, to help them roll better on road, and more tread on the shoulders so they don't wash out when your corner off-road. If you have a good pump with a gauge, you can improve tyre performance by changing the pressure. On road, use the maximum number that's printed on the sidewall; off-road, try the minimum.
Multi-use hybrids use mountain bike gearing but with bigger chainrings: for example, 48-36-26 rather than an off-road bike's 42-32-22. This reflects the higher speeds that a hybrid will be ridden at on road. The overall gear range is still good compared with most commuter bikes, so hills and heavier loads shouldn't be a problem. If you think they might be, look for a cassette that goes to 36-teeth rather than 32.
Most multi-use hybrids use disc brakes, like mountain bikes. These are powerful in all weathers and won't wear your wheel rims like rim brakes. Look for hydraulic rather than mechanical discs; they require much less lever effort and less maintenance. Chain-stay mounting is better than seat-stay mounting for the rear disc brake, as it won't foul a rear rack or pannier.