An LOI is a nonbinding legal agreement that not only expresses interest, but typically details the initial terms for the contemplated transaction, timing for due diligence, any contingencies (financing, Board approval, etc.), and specifies the timing to execute a final definitive agreement and close the transaction.
Secret: Time is NEVER on your side when you are buying. Never forget that. Keep in mind that if you are looking to explore the viability of a purchase, a good starting point is the LOI. Once you get beyond the "overview" of what you and the seller are looking for, detail everything out in the purchase contract, and keep all of the above in mind when doing so.