As one of my former bosses used to say, If it ain't in writing, it ain't. Many times I've been asked about the need to put understandings or agreements with partners, fellow shareholders, employees, customers etc. into writing. While for the most part verbal agreements have the same legal force and effect as written ones, they are much tougher to prove and therefore, to enforce.
Put it in writing. Fancy legal terms are not required (but may certainly help). Ask yourself, What is it we are agreeing to, and in the simplest, clearest language put it into a confirming letter, memorandum signed by the parties, even an email is better than nothing. In my view, email has been a boon to the extent it compels people to organize their thoughts, be concise and informative, and put things in written form that can be saved, reviewed later, etc.
Depending on what's at stake, and to be on the safest side, ask an experienced lawyer to review the writing before you send it. Ask if you can email it to him/her in draft form. An ounce of prevention is worth a pound of cure. As time goes by, memories fade, disagreements arise, you'll be glad you took a few moments to "put it in writing."