An estate plan is an umbrella term that refers to a plan as to what happens when someone dies or is incapacitated. Issues to consider are where your stuff will go, who will take care of your kids, who will make my decisions if you can't make them, how to make the probate process quicker and less expensive, continuation of a family farm or business, and what the effect of taxes and state liens will be.
There can be many pieces of an estate plan depending on the needs of an individual. Something that just about everyone should have is a will. Without a will, the state will apply the fallback provisions in state law to determine who gets your property, which may or may not follow your intent. Additionally, dying without a will which can specifically call for an expedient probate process, can cause a longer, more expensive probate process.
Also, everyone should have a health care directive, which states who makes the health care decisions for you in the event of your incapacitation and gives direction as to your wishes. This health care directive needs to include a HIPAA waiver to allow the hospital to release protected medical information to this person.
In the same vein, everyone should consider a power of attorney who can deal with your financial affairs in case of your incapacitation. However, once created, that person has as much right to your finances as you do and you should only execute a power of attorney if you have a trusted individual in place.
Besides these documents, other documents can be executed, trusts established, and business entities created which address your specific needs. This is why it is so important to contact a licensed attorney to look at all of your circumstances to craft an estate plan to fit your needs.