Basically, without rewriting volumes of books, a will is a document disposing of a person's estate after death by way of the terms of the written document as carried out by an executor, the process of which is overseen by probate court and probate court rules and statutes. A trust, on the other hand, where executed properly and where the full and complete estate is conveyed into the trust, allows for the administration of the person's estate after death by a trustee, who need not go through probate.
This answer is provided for informational purposes only. Actual legal advice can only be provided in an office consultation by an attorney licensed in your jurisdiction, with experience in the area of law in which your concern lies.
The above is certainly true, but a trust can also do a number of other things. For example, depending on the maker's needs and goals, a trust can be used for estate tax planning, looking after the needs of a disabled person, or managing funds for a child until he's old enough to manage them.
A will is important to manage your affairs after your death. The will nominates an executor to do that job. The executor is responsible for finding all the assets that may not be owned by a trust, settling up with your creditors and paying off the just debts, representing your interests in law suits, and so on.
A good estate planning attorney can review your situation and help you set up a structure for ownership and management of your assets so that your goals and needs are met, and that the documents are tailored for those purposes.