A relatively new category of digital cameras is the high-zoom compact. These share most of the same features as other compact cameras, but are often physically slightly larger and are fitted with longer zoom lenses, usually of around 7x to 10x magnification. The lenses are very compact for their power, and usually fold flush with the camera body. Many also have 28mm wide-angle capability. High-zoom compacts offer more versatility than standard compacts, making them very popular as travel cameras. Inevitably longer and longer lenses are being fitted to this type of camera, resulting in a certain overlap with the next category.
Beyond standard compact cameras we have what are sometimes called bridge cameras, because they span the gap between consumer snapshot models and digital SLRs. Other common terms for them include super-zoom or semi-pro.
This is a smaller category, but still encompasses a wide range of models from nearly all of the major manufacturers. Generally they have more powerful sensors then compact models, ranging from ten to 12 megapixels, and have powerful zoom lenses, currently ranging from 15x to 26x magnification. Nearly all super-zoom cameras have a full range of manual exposure controls and other advanced features such as image stabilisation, high-ISO sensitivity, adjustable flash output, adjustable focus points and multiple metering options. Some also have the ability to mount an external flashgun. They are, unsurprisingly, usually a lot more expensive than compact cameras, in fact some high-end bridge cameras are now more expensive than entry-level DSLRs.
The fourth category is the digital SLR, usually abreviated to DSLR. Five or six years ago these were the exclusive preserve of the wealthier enthusiasts and professionals, but thanks to keen competition between the manufacturers and increasing popularity the price of entry-level DSLRs has tumbled, and it is now possible to buy a good basic digital SLR complete with a standard wide-zoom lens for around £250, and even mid-range models are under £450. Only the very top professional cameras such as the Canon EOS-1DS MkIII and Nikon D3x still cost thousands, but then we've all got to have something to aspire to, haven't we?
All digital SLRs have a full range of manual functions, and generally have a much wider range of control than even the best bridge cameras. DSLR sensors range from six megapixels up to over 25 megapixels, and are physically larger than the sensors in compact and bridge cameras, giving them a number of advantages in terms of image quality. All digital SLRs have interchangeable lenses and a wide range of accessories, such as external flashguns, extra battery packs, WiFi transmitters and other gadgets. Most of the main camera manufacturers now make SLRs, includinge Canon, Fujifilm, Leica, Nikon, Olympus, Panasonic, Pentax, Samsung, Sigma and Sony.
Beyond these categories there are professional digital camera systems based on large and medium-format cameras such as Hasselblad and Mamiya, as well as specialised high-end gear with huge high-resolution sensors. They are quite breathtakingly expensive (in the order of tens of thousands of pounds) and are really outside the remit of this website. If you're considering one of these then you probably know more about them than I do, and if you don't then you really should start learning.