Vinoo Mankad was an Indian cricketer, an all-rounder who bowled left-arm orthodox. He is particularly in/famous for two things: one, salvaging a down-trodden India on at least one occasion with both bat and ball; two, while bowling, running out Australian batsman Bill Brown before bowling the ball. Brown had left his crease to "back up", or to take an extra couple of steps just after the bowler bowls the ball. A common occurrence normally, except that Brown was doing it far too early (in fact, before the ball was bowled). It is completely legal to run someone out in this fashion, and it has now been given the colloquial name "Mankad" after its perpetrator. However, it sparked a controversy in the cricketing world at the time (late 1940s).
Why? Supposedly it is unsportsmanlike and unscrupulous to run someone out in this fashion, namely, before either batsman has had a turn1. Indeed, it is common convention now for bowlers to warn batsmen that they are backing up too early before they actually Mankad him (it is noted that Mankad warned Brown at the time). As with most controversies, the practice has had its defenders and detractors - the majority being detractors AFAIK - with defenders claiming that the Mankad is within the laws of the game, and that the batsman is trying to gain an unfair advantage.
The practice is far more common and far more accepted inside than outside, and as I have played just as much cricket indoors as outdoors, I am a defender of the practice on the oval. If you can't effectively Mankad inside, you've given the opposition at least five free runs. Given that five runs is often some 7% of the total score, it's a big difference. In addition, I agree with the unsportsmanlike conduct of the non-striker. As I say, it is common to back up - in fact, if you don't back up, you can't play at Test level - but backing up too much is giving the batsman too much room to run. Borderline cheating. YMMV but I stand by my position.
1(Outdoor) Cricket is a turn-based game. Before the ball is bowled, the batsman can analyse the field and predict which ball the bowler is going to bowl, so that his "turn" of less than a second can be used effectively. Additionally, the non-striker gets a "turn" of 5-20 seconds, to make runs in conjunction with the striker. This is a poor explanation but I can't think of anything better.