Even if you don't have family members in the military, you've surely seen these in a movie or TV show:

Image: Catherine Lowrey

Sometimes jokingly referred to as "chest candy" by soldiers, those are service ribbons, also called ribbon bars, and they're essentially a UX concession to troops. To explain, a soldier may be awarded a medal such as the Purple Heart, which looks like this:

Image: Jonathunder, CC0

During ceremonies where medals are awarded, soldiers may be wearing battle dress uniform…

In 2009, Staff Sgt. Beau M. Martindale is awarded the Purple Heart by Maj. Gen. Yves J. Fontaine during a ceremony on Coleman Barracks. Image: U.S. Army. Spc. Adrienne Killingsworth, 18th MP Bde. PAO

…or civilian clothes, if the medal is awarded after their service has been concluded:

In 2010, Congressman Christopher Smith presented the Purple Heart Medal to Tuskegee Airman Tech. Sgt. (Ret.) George Watson Sr. with then Col. Gina M. Grosso, Joint Base McGuire-Dix-Lakehurst commander. The medal was awarded 66 years after he sustained the injury warranting the medal. Image: Wayne Russell

In those ceremonies, it's not difficult for the soldier to have a single medal pinned to their clothes. However, soldiers can receive medals for personal achievements, unit achievements and for serving in particular regions. For instance, soldiers who served in Afghanistan for 30 consecutive days (or 60 non-consecutive) are awarded the Afghanistan Campaign Medal, which looks like this:

Image: Defense Logistics Agency

Because soldiers can rack up a lot of medals, wearing all of them at once would be unwieldy (and noisy). Imagine you've got all of these:

Image: USA Military Medals Rack Builder

Thus when soldiers are wearing service dress uniforms—essentially their office clothes—the medals are represented by the more compact service ribbons.

The design of the service ribbons is simple. Medals technically consist of two parts: The suspension ribbon, and the planchet (the metal part).

For visual clarity, the suspension ribbon is the easier one to scale down. Thus the service ribbons are simply reductions of the suspension ribbons:

Afghanistan Campaign Medal service ribbon. Image: Ipankonin

Afghanistan Campaign Medal, suspension ribbon left, service ribbon right. Image: Staff Sgt. Alexx Pons

Purple Heart Medal. Image: USAF

Purple Heart service ribbon. Image: United States Army Institute of Heraldry

These service ribbons—whose size is regulated at 1 3/8" by 3/8"--are much more practical to wear on a service dress uniform.

Image: Mass Communication Specialist 1st Class Chad J. McNeeley (United States Navy)

It should be noted that while service ribbons started out as practical reductions of suspension ribbons, these days there are some service ribbons that do not have corresponding medals and suspension ribbons; those service ribbons are the entirety of the award themselves.

Here are some examples of service ribbons from the various U.S. Armed Forces:

You can see a complete list of them here.