Defense
Defense in Japanese mahjong focuses on one important concept: avoiding other players' hands, by not discarding winning tiles. Other concepts rests on not giving other players any tiles needed to call. This aspect of the game is greatly emphasized during other player(s)' riichi declarations and/or threat of high scoring hands. Regardless, it is in the best interest for players to avoid losing points directly to other players, as much as possible.
General overview
Defense is part of the game's learning curve. Typically, beginner players may not be aware of defensive play, due to greater focus devoted towards general hand development and memorization of yaku.
Suji
Suji focuses on taking advantage of the furiten rule, and its application to the mahjong octaves.
On the other hand, this strategy may actually work backwards, when suji is actually used to bait players into discarding tiles that appear to be safe.
Kabe
Kabe takes into account the number of tiles of particular types unavailable for general play. In general, these are tiles in the discard pile, tiles used for open calls, and any tiles used as dora indicators. In other words, these are the visible tiles. When applying machi (wait patterns), players developing particular patterns must also take into account the number of tiles left available. In general, patterns waiting for three tile types or more are considered to be very powerful waits. However, that "power" may actually be diminished, if a significant number of tiles are made unavailable. For example, ryanmen (open waits) may actually turn into "penchan" (end wait), when all four of one side of the ryanmen is no longer available. Basically, as the number of discards increase, the number of tiles used for hands decrease.
Betaori
Betaori focuses on discarding nothing but safe tiles. In this state, a player has completely abandoned all hope of developing a winning hand; and instead, the hand is dedicated towards avoiding any threatening hands. At times, even a player achieving a tenpai hand may need to completely break the tenpai for avoidance.