A general power of attorney document is used to give some trusted individual broad authority to make decisions on your behalf. It is commonly used to establish a recognized authority for decision making when you are expected to be unable to handle your own affairs, either due to prolonged absence, such as through travel, or incapacity due to planned medical procedures. Though it can be used to give someone the power to make medical decisions for you in an emergency situation, the general power of attorney goes far beyond this particular situation.
The agent designated in a power of attorney to act on your behalf does not necessarily have to be a lawyer. As attorney-in-fact, they are authorized to act for you, as you, and make legally binding decisions, but they are not legal counsel and cannot represent you in court. A general power of attorney is one that places no specific limitations on the scope of the agent's powers to act on your behalf as attorney-in-fact, but would generally be automatically revoked, unless stated otherwise, in the event of mental incapacitation.
The term "general" relates to the scope of the powers associated with a general power of attorney. These generally include banking transactions, access to safety deposit boxes, buying and selling stocks and real property and exercising related rights, managing life insurance policies, entering contracts, filing tax returns, and handling pensions, retirement accounts and government benefits. A power of attorney can be revoked at any time, and must be designated as "durable" to persist if the principal becomes incapacitated (thereby losing the ability to revoke) by an accident or sudden illness.
There are two main types of power of attorney, relating to when and how they take effect. A springing power of attorney is one that takes effect only upon a necessary condition, such as mental incapacity, or upon a predetermined date. Because of the broad powers associated with a general power of attorney, most are springing, and intended to grant sweeping authority when the principal is unavailable. Powers of attorney that are not springing, which take immediate effect and generally persist indefinitely, are usually much more limited in scope.
A general power of attorney are understood to revoke any previous powers of attorney, especially where the two might overlap. Usually such an express revision is made explicitly in the document to avoid confusion in situations where clarity of authority is needed most. If the intention is not to revoke other powers of attorney, or to only grant authority over specific matters, there might be documents better suited to your purpose than a general power of attorney.