Your word should be good for something, and your reputation in the community is at stake. Decide on each job offer for what it is and has to offer. It is standard practice to formalize an acceptance of a job offer verbally. However, it is highly recommended to always get a written confirmation of the job offer which spells out job title, starting date, and salary information, etc. In most cases, this written confirmation can be a legally binding document.
Some organizations require the signing of a contract.
Read through everything, including the fine print, before signing. Make copies for your files. Once you have accepted a job offer, write to those employers who are still considering your application and inform them that you are withdrawing. This action is a matter of courtesy and is a way of protecting your reputation with the community. At a future point, you might be considering employment with these organizations.