If your grandfather retired as a Major then yes, he was an officer.
A career in the armed forces may be very very rewarding. You have job security, great benefits, a structured life, and you are working for the benefit of your country.
Here is a list of enlisted job fields in the Marines (if you follow the link you can check out the jobs for the other branches as well). As you can see, there's a TON of different kinds of things you can do in the armed forces.
01 -- Personnel and Administration
02 -- Intelligence
03 -- Infantry
04 -- Logistics
05 -- Marine Air Ground Task Force (MAGTF) Plans
06 -- Communications
08 -- Field Artillery
09 -- Training
11 -- Utilities
13 -- Engineer, Construction, Facilities, and Equipment
18 -- Tank and Assault Amphibious Vehicle
21 -- Ground Ordnance Maintenance
23 -- Ammunition and Explosive Ordnance Disposal
26 -- Signals Intelligence/Ground Electronic Warfare
27 -- Linguist
28 -- Ground Electronics Maintenance
30 -- Supply Administration and Operations
31 -- Traffic Management
33 --Food Service
34 --Financial Management
35 -- Motor Transport
41 -- Marine Corps Community Services
43 --Public Affairs
44 -- Legal Services
46 -- Combat Camera
55 -- Music
57 -- Chemical, Biological, Radiological and Nuclear Defense
58 -- Military Police and Corrections
59 --Electronics Maintenance
60/61/62 -- Aircraft Maintenance
63/64 --Avionics
65 --Aviation Ordnance
66 -- Aviation Logistics
68 -- Meteorology and Oceanography
70 -- Airfield Services
72 -- Air Control/Air Support/Anti-air Warfare/Air Traffic Control
73 --Navigation Officer/Enlisted Flight Crews
80 -- Miscellaneous Requirements MOSs
You can check out all of the specific jobs inside of the job fields here:
http://usmilitary.about.com/od/enlistedjo2/a/marinejobs.htm