The most common format was like this: (more formats will be added soon)
LINE 1: TOM J SMITH
LINE 2: 12345678 T42 A
LINE 3: BLANK
LINE 4: BLANK
LINE 5: 14 spaces and then religion (C, H, or P)

The 8 digit number is the Army Serial Number. It is laid out like this:
FIRST DIGIT(S): 1 for Regular Army, 3 for Draftees, 20 for National Guard
SECOND DIGIT(S): Corps Area/Service Command where inducted:
Hawaiian Dept-01
Panama Canal Dept-02
Philippine Dept-03
Puerto Rican Dept-04
First Corps (Maine, New Hampshire, Vermont, Massachusetts, Rhode Island, Connecticut)- 1
Second Corps (New Jersey, Deleware, New York)- 2
Third Corps (Pennsylvania, Maryland, Virginia, District of Columbia)- 3
Fourth Corps (North Carolina, South Carolina, Georgia, Florida, Alabama, Tennessee, Mississippi, Louisiana)- 4
Fifth Corps (Ohio, West Virginia, Indiana, Kentucky)- 5
Sixth Corps (Illinois, Wisconsin, Michigan)- 6
Seventh Corps (Missouri, Kansas, Arkansas ,Iowa, Nebraska, Minnesota, North Dakota, South Dakota)- 7
Eighth Corps (Texas, Oklahoma, Colorado, New Mexico, Some of Arizona)- 8
Ninth Corps (Washington, Oregon, Idaho, Montana, Wyoming, Utah, Nevada, Some of Arizona, California, Alaska)- 9
 
REMAINING DIGITS: Sequential number.  I recommend using your dirth date MM/DD/YY as the 6 digits.  That way they are easy to remember.

More dog tag information:
Civil War Tags
Spanish American War Tags
WWI Tags
WWII Tags
Korean Era Tags
Vietnam Era Tags
Current Issue Tags
Tags of the Future?
Medical Warning Tags

If you have any information to add to this section, contact Robert.