1) Computer Specs:
Your computer is likely either a Mac or a PC and you will need to look for a webcam that is compatible with your Operating System. If you have Windows 7, look for a camera that is compatible with it. Often this is listed under Operation Requirements as something like "Windows XP or Vista and above".
Also, depending on the quality of the camera and the software that accompanies it, there is a chance that you'll need to have a laptop with a certain amount of memory free as well as a certain amount of RAM memory for the camera to run. (Cheaper models - of which there are many and quite often of high quality too - won't demand too much in this regard.)
You will also need a USB port or equivalent free to connect the camera to the machine. Make sure to check that your computer meets these requirements before purchase.
2) Picture/Video Quality:
Many webcams traditionally shoot in a frame ratio called 4:3, which is the same ratio that old, square analogue TVs broadcast in. Now, in the age of widescreen we are used to seeing things in 16:9. If you are the kind of person who vlogs, it is particularly tempting to want something that shoots in 16:9. Most external webcams provide this, but be certain to check the Product Specification before purchase to make sure. There's also a difference between HD and SD. SD or standard definition shoots at 480p or below and can have a lower resolution of picture than a camera that shoots in HD. High definition is 720p or 1080p and has a higher resolution. HD webcams will shoot in 16:9 ratio.
3) Basic Features:
The most basic things you can expect from a webcam are things like autofocus and a form of evaluative metering.
Autofocus means that as you move around in the frame of the camera, the webcam automatically focuses the screen on you to keep a nice, crisp image and stop unsightly blur and motion blur.
Evaluative metering is very rarely referred to as that: rather, many manufacturers choose to call it things like ClearFrame or ScreenAdapt or something silly like that that they can attach a little TM symbol to. Really this just means that if you were to flick a match in front of the camera, instead of the screen flaring into white because of the sudden onslaught of bright light, it automatically adapts to the change in light to keep a consistent appearance.
You should also expect some form of software that allows you to play around with the brightness and contrast and enable you to record and playback your footage. (NOTE: You may also expect that webcams come with a microphone or sound recording ability, but some don't so always check the product spec.)
Most webcams should also come with a type of attachment that connects it physically to your computer. Some of these attachments are more versatile than others so if you want a camera that can be placed on a nearby surface but not on your actual computer, be sure to check what its attachment is like. Make sure to check that all of these features are present by reading the product spec and customer reviews.
3) Extra Features and Special Requirements:
If you need to record to an external device or if you wish to use your webcam to record video but an external microphone to record audio, be sure to check that your product can do this. It is harder to gauge from Product Specs whether or not this is possible, so it might be worth popping into your local computer store or giving the manufacturer a ring once you've gotten a good idea of which model you'd like to buy.
Some webcams also come with more extensive software - editing software, fun software - and some have the ability to upload directly to YouTube from within the product itself. If this is a feature you'd like, it is worth spending extra cash for better software. All softwares, however, are different so make sure to check and read up on the model you are considering buying to make sure it fulfils your needs.
Considering how cheap many HD webcams actually are, you might wonder why it is so important to consider all of the above features before purchasing a camera. Truth be told, you may be very happy with any camera that you buy - however, there are so many great and cheap options on the market that you may as well shop around for something that fits your exact needs.
Some cameras are as low as £20 and some are more than £100. How much you choose to pay depends on several factors - how much you use your camera, what kinds of features you need, how versatile you would like your camera to be etc. If you are considering spending up to £100 you may wish to look outside of webcams and instead start browsing camcorders, because for not too much more money, they offer great versatility and durability. If, however, you are happy with a webcam model under £90 or so, you may want to start narrowing down your specific needs to fit price. For example, a 720p HD cam might cost you about £30, but a 1080p HD cam up to £48. If you think it is worth paying that much more for higher quality picture and a select few upgraded features, then you may as well buy the higher option. But if you use the webcam more sparingly, choose the lower option and save yourself some cash. The features will still be high quality and the camera will still be imressive for your needs.
I hope this basic guide has given you some ideas on what to look for when selecting an HD webcam and I hope you find something that fits all your needs! Good luck.