A General (or ordinary) power of attorney (POA) is the most straightforward POA which can be made. It's often used for one-off events and should be for limited periods of time only. It allows you to appoint someone to be your 'attorney' - usually a friend or relative - which gives them the legal authority to sign documents and manage affairs on your behalf. This can be useful in the situation where an important transaction like exchanging contracts on a house is taking place if, for example, you're going to be abroad or in hospital. It's important that the person who is appointed to be your attorney is someone who will act in your best interests as the POA will give them the ability to make decisions and deal with your affairs on your behalf. For further information, read General powers of attorney.
You can appoint more than one attorney (as long as they are over the age of 18 and not bankrupt) but you'll need to set out the decision-making process in advance (ie do they need to act jointly or severally). Once you no longer require the POA or if you become unhappy with one of your attorneys, you should revoke it immediately using a Deed of revocation. For further information, read Revoking a power of attorney and lasting power of attorney.