Personally mine were:
Fathers side - Captain in charge of a field gun in the North Africa/Italy campaign and a nurse
Mothers side - commander of a New Zealand destroyer [great grand father] and an infantry man in the soloman Islands campaign
My great-grandfather on my mothers side fought in the war. He left his tank to have a cigarette and then the tank got blown up with all his mates/colleagues inside of it. Lived the rest of his life with a **** load of shrapnel in his leg.
Paternal: Grandfather was an admiralty clerk, and his brother flew Liberators in the Far East. My Grandmother was younger, and was an evacuee.
Maternal: I don't know about my grandfather, but my grandmother's brother was a partisan fighting against Mussolini (this side of the family is Italian), and was killed.
Regarding World War One, my paternal great-grandfather was a member of the Territorial Force (akin to the TA nowadays), and fought in the opening stages of the war, like at Mons. He was captured during the Somme offensive of 1916, and spent the rest of the war as a POW. His brother was killed in either 1916 or 1917. My other paternal grandparent and uncle were in the Navy, and at least one of them was killed during the Battle of Jutland. On my mother's side, my great-grandfather was in the Italian army fighting the Austrians, and he was decorated for doing something (I'm not quite sure what), which resulted in him losing an eye, and one of my cousins actually own his glass eye...we also have a framed certificate and medal for this, which has the stamp-signature of Mussolini on it.
My paternal grandmother was too young to participate but she lived in Austria during the Anschluss and remained there during post-war Allied occupation, where I believe she was in the Russian zone, before moving to Britain in the late 1940's.
My paternal grandfather was born in Western Ukraine, in a village called Tyrszakiv not far from L'viv. And following German occupation, my grandfather was one of 'ostarbeiter' or Eastern workers who volunteered to work in a German labour camp. He had three brothers, one who enlisted in the Red Army and died in Russia somewhere, one who joined him working in a German labour camp and another who remained in Tyrszakiv as an Organisation of Ukrainian Nationalists sympathiser.
My maternal grandfather never spoke of what he did in the war. All we know is that he served in Africa.
My maternal grandmother worked in the factories, assembling planes.
My grandpa was 13 when World War II started and my grandma was even younger, so I don't think they did anything special. My other grandparents were a little older, and I think my grandpa was a soldier, and he befriended a monkey or something. I don't really remember because he showed it to me ages ago, and he's since passed away.
World War I is much more interesting for my family. My great-great uncle (I think) was this guy.
My Paternal Grandfather (my father's father) Was in the Royal Ordinance Corps. By the time war broke out and he joined up he was in his late 20's. Because he was in his late 20's he was given more responsibility and he quickly rose up the ranks. When he was demobbed after the war ended he was a Warrant Officer First Class, which is basically Regimental Sergeant Major.
He was all of the place but mostly was based in Alexandria in Egypt.
My Maternal grandfather (my mother's father) was in an infantry regiment. I think it was called "The Kings regiment" or something like that. It was one of the ones disbanded either right after the war or during the big tidy up the army did in the 50's where a lot of regiments were either merged or disbanded.
He fought all over including Italy, Greece, and in North Africa. He fought at the battle of El Alamein. I think he was a Corporal.
My grandfather on my mother's side was an RAF engineer in Burma and my grandmother worked in a munitions factory.
On my dad's side, both grandparents were too young to serve (both were born in 1931). My grandfather was evacuated to Teesdale while my grandmother remained in her native Hebburn, Tyneside for the whole war. I'm very lucky that before they died, two of my grandparents wrote their memoirs, describing in detail their experiences of the war. My mum has all my grandfather's diaries from his time in Burma as well. Fascinating to read.
I also know that my grandmother's cousin was killed in the sinking of HMS Hood by the Bismarck.