Level:
GS - Grand Slam
MS - Masters Series
CS - Championship Series
WS - World Series
OL - Olympics
Ch - Challenger
F - Futures
Sa - Satellite
DC - Davis Cup

Opponent Rank refers to a player's opponent's entry ranking as of the date of the event.  "NR" means the player was "not ranked" at the time of the event.

Davis Cup:
A lot needed to be shown to identify the Davis Cup ties so some short-hand was used.  For Davis Cup results (noted by a "Level" of "DC") the "Tournament" was noted by the following format:
Zone Group: Matchup

Zones:
World Group
A/O - Asia/Oceania
Am - American
E/A - Euro/African

So, for example, the following matchup
E/A IV: ZAM d. SMR
means, "Euro/African Group IV: Zambia d. San Marino"

For a list of all country codes, take a look at this Excel spreadsheet.