All this data is potentially out of date, and should be taken with a truckload of salt
"Suicide ganking" refers to the practice of intentionally attacking enemies in high-security space, with the expectation of losing one's ship to Concord. Some targets have very valuable items equipped and/or very valuable items in their cargo, and killing them and recovering the items that are dropped will pay for the costs incurred in losing the ships necessary to kill it. Concord response times are longer in lower-sec systems, so suicide ganking is generally most profitable in 0.5.
Suicide gankers usually have at least two alts, hanging at a gate with intensive commercial traffic, usually in a 0.5 (or higher-security) system. The first alt is the actual ganker who selects potential victims by scanning all "promising" ships (Industrial, Transport, Freighter) with a Cargo Scanner. If the items shown on scan are valuable enough for the ganker to break-even, he/she commits the suicide assault. The ganker has relatively short time to evaluate the contents of the cargo bay and to assault the target, hoping to destroy it before Concord destroys his/her own ship. A low-cost T1-fitted battleship could alpha-strike typical Industrial ship. For taking-down a bigger ship (like Freighter), the Suicide gankers usually form fleets with multiple battleships in order to deliver more DPS and destroy the target before Concord wipes them out. The second alt plays the role of a looter / salvager / hauler who collects the items dropped from the desroyed ships as quickly as possible and then in some cases salvages the wrecks. This is usual practice, because obvously, the attacking alt(s) end-up in Pods after Concord's intervention and could not collect the loot by themselves.
As of EVE Online Retribution, released 2012-12-04, kill rights are now created when engaging a ship illegally in high security systems or a capsule in low security systems. This means that even failed suicide ganks will generate kill rights.
If you attack an Iteron Mark V full of a billion ISK worth of stuff, you expect about half of it to drop when the ship is destroyed. If you lose a battleship fit with cheap gear, you will easily come out with a profit.
The risks are higher if most of the value of the cargo is in one or two stacks of items, as each stack either survives intact or is completely destroyed.
Your security status drops for each attack you make, and this, rather than ISK, is often the limiting factor, as it takes a long time to regain the required status to be able to re-enter high-sec without being continually shot at by the faction navies.
Suicide ganking was a popular strategy until recently, both with GoonSwarm's Miniluv campaign and with several groups of pirates who did it for profit. However, recent changes in game mechanics - faster Concord response time and higher security status losses for criminal actions in highsec - have largely eliminated the practice. It's still possible, and still practiced occasionally, but is much less common than it used to be.
Attention returned to the mass suicide gank with Hulkageddon.
There was a major change in the Crucible expansion which affects suicide ganking. Insurance will no longer be payed out to ships which are destroyed by CONCORD, hence suicide attacks will become a bit less profitable. CONCORD are not obliged to pay someone insurance for comitting a criminal act, hence the change to their policies.