What were the reasons behind the Soviet invasion of Afghanistan? Was it simply for territorial gain or was there stuff in Afghanistan that they wanted? Was it just to piss off the US?
Not the US.
China.
Think of the idea from Chess—Centerboard Control.
As in “control of Central Asia”.
Short partial answer: Their client state that they had established on their border looked like it might collapse, with a loss of prestige, security (would it be a U.S. ally? Saudi?), and possibly encouraging their own USSR Muslim populations to revolt
Longer answer, they were sucked into it step by step:
King Zahir’s cousin, Sardar Mohammad Daoud, charging corruption and malfeasance against the royal family and poor economic conditions seized power in a military coup in 1973. Daoud abolished the monarchy, and declared Afghanistan a republic with himself as its first President and Prime Minister. Daoud was unable to make any real economic progress and his coup just fed political instability.
In 1978 the PDPA (People’s Democratic Party of Afghanistan aka The USSR-backed Communist Party of Afghanistan) overthrew the Daoud govt (& killed him).
From FAS http://www.fas.org/irp/world/afghan/intro.htm
Opposition to the Marxist government emerged almost immediately. During its first 18 months of rule, the PDPA brutally imposed a Marxist-style “reform” program which ran counter to deeply rooted Islamic traditions …
… The Soviet Union moved quickly to take advantage of the April 1978 coup. In December 1978, Moscow signed a new bilateral treaty of friendship and cooperation with Afghanistan, and the Soviet military assistance program increased significantly. The regime’s survival increasingly was dependent upon Soviet military equipment and advisers as the insurgency spread and the Afghan army began to collapse.
By October 1979, however, relations between Afghanistan and the Soviet Union were tense as Hafizullah Amin refused to take Soviet advice on how to stabilize and consolidate his government. Faced with a deteriorating security situation on December 24, 1979, large numbers of Soviet airborne forces, joining thousands of Soviet troops already on the ground, began to land in Kabul under the pretext of a field exercise. On December 26, these invasion forces killed Hafizullah Amin and installed Babrak Karmal, exiled leader of the Parcham faction, as Prime Minister. Massive Soviet ground forces invaded from the north on 27 December 1979.
the broader reason was the strategic position of Afghanistan. That’s why they wanted it in the first place.
Bosda said
What, exactly, was the strategic value. I understand when the US wants a Pacific rock for strategic reasons. We can refuel planes, etc.
What can you cite that would say that was their primary objective over a reason as old as the czarist empire—as a buffer against other countries in their sphere of influence…
as jimmmy offered? Simple control of all of their border states and their Muslims would be enough reason.
WAG: easier access to Middle Eastern oilfields?
(yes, just a WAG, based on nothing more convincing than an old Harold Coyle novel)
Isn’t it obvious? They did it to goad the US into boycotting the 1980 Olympics, so they could win all the big events. 
I remember hearing people mention that unlike the USSR, Afghanistan had a warm-water sea shore.
Only if they took over Pakistan or Iran after Afghanistan. Afghanistan was, and remains, landlocked.
My bad – next time, I’ll look at a map.
Shakespeare, you are half right. A big factor in the USSR’s campaign of Afghanistan was to obtain a warm-water port. Of course, Afghanistan is landlocked, so it would have been a jumping-off point for taking over Pakistan or Iran at a later date.
Source: some History Channel program I saw many years ago. 
[brief hijack]
Can anyone recommend a good, reasonably balanced history book on Afghanistan, preferably one that won’t put me to sleep?
[/brief hijack]
For the complete answer to why the USSR invaded Afghanistan, you should check out the news archives from the time, covering the debate in the UN:
Because the Soviet Union had seen how badly the US had fucked itself over in Vietnam, and they wanted to try the experience for themselves, but rather then go to war in a country in say South America, they chose someplace a bit closer to home.