"You didn't happen, by any chance, to see who broke into Snape's office, did you? On this map, I mean?”
-- Professor Moody (aka Barty Crouch Jr.) to Harry Potter (GF25)
If one follows the narrow stone staircase leading from the Entrance Hall to the dungeons, the entrance to Snape’s office is halfway down the cold passageway leading from the foot of that staircase (CS5). Snape’s office is adjacent to the Potions classroom, or at least close enough that Hermione could slip from one to the other and back without being spotted during a commotion in class during her second year. By Harry’s fourth year, Snape had a charm on his office that “none but a wizard could break” which would have prevented Peeves from getting in and meddling with his possessions, although Dobby was able to get in and steal gillyweed from Snape’s stores anyway.
Snape is in the habit of keeping the place dimly lit, judging from Harry's visits during his second (CS5), third (PA14), fifth (OP24 and other tutorials), and sixth years. Snape has a fireplace (which is sometimes lit) that is apparently connected to the local Hogwarts Floo network, since he was able to call Lupin (then working at Hogwarts) through the fire, and Lupin was able to travel through the fire to Snape's office.
The walls, which are in shadow, are lined with glass jars, including the wall behind Snape's desk. Each jar contains some potion (differently coloured in different jars) in which is suspended slimy bits of some animal or plant (different ones in different jars). For example, one jar contains a large dead frog suspended in some purple liquid (OP26). Snape also keeps a cupboard in one corner containing his private stores of potion ingredients. Snape also keeps a wall clock, occasionally an extra table, and a chair or two for visitors (HBP24).