A will is a formal set of instructions to your loved ones and the court telling it what to do with your kids and your stuff after you pass away. If you do not have a lot of assets and you do not anticipate your family fighting over anything, a will may be a good choice.
To enforce a will, a probate lawsuit needs to be filed. Probate cases take a lot of time and can be expensive. It also makes a lot of your private information publicly accessible. If you end up with a messy contested probate case, a lot of money that you wanted to pass on to your family will end up going to lawyers instead of your family. Many families create a trust to avoid this.