A service medal is a military award generally of the lowest degree. A service medal is awarded to a member of the military who joins the military, or is already serving, during a designated time period. No other requirement is needed to obtain the service medal other than a person must simply be a member of the armed forces.
Campaign medals are similar to service medals (and are sometimes even referred to as service medals), however the main difference is that a campaign medal is awarded to a military member who participates in a military operation whereas a service medal is blanket awarded to everyone in the military regardless of campaign credit.
An example of a service medal currently issued by the United States armed forces is the National Defense Service Medal which is presently automatically awarded to a service member upon their first day of Active Duty. The Global War on Terrorism Service Medal is another decoration which is presently awarded to any active duty member of the United States military (who has served after the September 11 attacks) after 30 consecutive days of service at a permanent duty station.