Usually, a surviving parent assumes the role of sole guardian of your minor children.  However, if neither spouse survives, or neither is willing and able to act, it is very important to name a guardian in your will.  The guardian you chose should be over 18 and willing to assume the responsibility.

A durable power of attorney is a legal document authorizing someone to act on your behalf.  You determine how much power the person will have over your affairs.  A general power of attorney authorizes your agent to conduct your entire business and affairs (buy & sell your property, pay taxes, invest your money in stocks, bonds, etc.).  A limited or special power of attorney authorizes your agent to conduct specified business or make only certain decisions on your behalf.   A power of attorney is considered "durable" when it remains valid even if you become incompetent or incapacitated.