A Memorandum of Understanding (MOU) is a short agreement in principle of the broad outlines of the deal, worked out mainly by the executives of the companies involved with reviewinput from senior legal professionals for each company. The MOU itself will often also clarify the general structure, which could be a sale of assets, or a sale of ownership interests in a company or a subsidiary of a company, and so on.
Usually, an MOU is followed by having lawyers for each party to the merger draftnegotiate a much more specific and detailed contract with lots of exhibits and scheduled that address deal specific terms. This will set deadlines for each step of the process, although it isn't uncommon to adjust those deadlines over the course of the deal. The final deadline will usually be a closing date. The final M&A contract will also usually contain a long list of representations of fact, with specifically listed exceptions (e.g. all accounting statements attached as scheduled to the contract are materially correct, there are no pending lawsuits except . . . , there are no pending government investigation except . . . , the company has no derivative contracts except . . . ).