Ok,
Basically, in cricket, the most telling statistic of a player's career is their average, which is how many runs they score for every wicket they lose (a wicket is like an out in baseball).
For a batsman, an average of 40 or over is considered fantastic. 50 or over is considered to be one of the greats of the game. 60 and over has only ever been achieved 4 times in the history of the game and it's 2,500 international players in 132 years, 3 of those averaged between 60 and 61, one was higher than 61: Sir Donald Bradman, who averaged 99.94.
I can't think of any other sport where one player has the key statistic of that game nearly double the next best.
A player scored 100 in an innings is a big deal. It is the key milestone in batting. Most batsmen who play 50 matches or so score 4 or 5, he needed 4 more runs in his career to AVERAGE 100
Records | Test matches | Batting records | Highest career batting average | Cricinfo.com
Those stats read left to right:
Matches (games played, two innings per side)
Innings
Not Outs (team all out with that person still in, or game won)
Runs (1 scored every time the batsmen change ends, 4 if it goes into the fence on the bounce (fence is approx 80 meters (90 yards) away, 6 for over the fence on the full)
Highest Score for one Innings
Average
100s
50s (noting that if a score of 100 or more is achieved, it doesn't count as a 50)
0s
The other thing that makes him so great is the conditions he played under. Technology has developed cricket in leaps and bounds since he played in the 20s, 30s and 40s. The bats used have been developed to be bigger, lighter and sweeter, rule changes have come in to favour batsmen, protective gear has improved (helmets being one example), and perhaps most importantly, pitches are covered when it rains. This is important because a wet pitch is far more difficult to play on. Bowlers bowl up to 100 miles per hour (160 KPH), and the ball deviates and bounces variably when it is wet. Bradman often played games with wet pitches, whereas today, batsmen never do.
In any pure contest about the greatest sportsperson ever, the true indicator surely has to be dominance over their contemporaries. Bradman has to win if that is the measure because noone has ever looked like coming close to his record