Simply put, a last will and testament (a "will") instructs those you leave behind what to do with your stuff upon your death. You may think nobody will want your stuff but think again. Often it is the things of sentimental value that loved ones will fight over when you are gone. Through your will you appoint an "executor" who has authority to take your will through the probate process and distribute your property to your beneficiaries according to your wishes.
More importantly, if you have minor or special-needs kids, your will nominates a guardian to care for these dependents. The death of one or both parents of small children is a tragic event. Having clear guardianship direction in a parent's will helps relieve some of the ensuing stress and confusion.
Obviously a will addresses tremendously significant issues and warrants attention. Fortunately, it can be fairly simple to create without much time or expense. But ironically, most parents with minor children do not have a written will. Do your loved ones a favor - create a will.
Lastly, I appreciate a DIY mentality as much as anyone but in this case it is best to talk to a lawyer to make sure your will is drafted, executed, and witnessed properly. You might save a few dollars doing it on your own but sometimes professional help is best. After all, if you needed an appendectomy you could probably read all about it on-line but is it something you would do yourself? Same idea.