execute (verb): To make valid, as by signing: execute a deed. To perform or carry out what is required by: execute the terms of a will. When a person "executes" a document, he or she signs it with the proper "formalities". For example: If there is a legal requirement that the signature on the document be witnessed, the person executes the document by signing it in the presence of the required number of witnesses. That's why "execute" is the term used in contracts.
EFFECTIVE DATE VS. EXECUTION DATE
The execution date is the date that the party signs the document. The effective date is the date that the agreement becomes effective and can be a specified date other than the date the agreement was signed. If no other date is specified, the contract is effective on the execution (signing) date.
JOINT AND SEVERAL LIABILITY
This means that all liable parties are equally and personally liable for the obligations of a judgment, loan, debt or other liabilities. Being Severally Liable means that, if one liable party is unable to pay its portion of an obligation then the remaining liable parties will be responsible for their own portion as well as this unpaid portion.
Source: http://wiki.lawdepot.com/wiki/Legal_FAQ
SERVING A DOCUMENT / PROOF OF SERVICE
To "serve" a document refers to the formal delivery of a document from one party to another in a manner that is legally acceptable. This may be accomplished in many ways including delivery in person, delivery by registered mail, delivery by process server, or for more serious matters delivery via a sheriff or bailiff. The person to whom a document is delivered may attempt to avoid or deny delivery of a document. It is prudent to use some method of verification of delivery.
Proof of Service is evidence that can be introduced into court to verify that the party did in fact receive a copy of a document.
A document may also be served by certified or registered mail.