A Real Estate offer can become a legally binding contract. Making an offer in a Real Estate purchase is much different than an offer made in other negotiations in which you many participate. If you offer to buy a house at a certain price and with certain terms, and the seller agrees and notifies you of their acceptance, you have bought a house! The closing and escrow details may still need to be finalized, but an offer can turn into a contract in a matter of hours, so it is important that you understand the potential consequences of an offer. Naturally, not being able to qualify for a loan to purchase the property will nullify the offer as will an inspection that falls short of expectations, but don't make an offer to purchase unless you intend to complete the sale.
Any verbal offers, counteroffers, or acceptances will be virtually impossible to legally enforce, since it would be nothing more than your word against someone else's. Written offers remove almost all doubt because they cannot become contracts (and therefore binding) until seen by, and accepted through signatures of all parties.
A Real Estate offer is more than simply an offer of price. All details of the eventual sales contract must be addressed in the offer. Some of the items which may be included are:
Sales price
Any concessions made by the seller
The amount of buyer's "earnest money" or deposit that accompanies the offer.
Financing contingencies (subject to you securing an acceptable mortgage)
Inspection contingencies (subject to an inspection report that is acceptable to you)
When it has been submitted to the seller, accepted by the seller, and you have been notified of its acceptance. Up until that point it remains only an offer or counteroffer, not an enforceable contract.
