A tech raid is a military intervention against another nation to plunder their technology supply. Most alliances view this behaviour as criminal and may refuse to take up a nation with a history of raiding. If the victim is in an alliance, he will probably receive assistance to fend off the raider(s).
Raiding is attractive to some as it's a cheap and fun way to grow your nation. Arguably, it's less efficient than growing peacefully because of the losses made in the case of a successful counter-attack, plus the military upkeep damages your income.
Spy attacks are a way to steal tech off a foreign entity without going to to war, but are also viewed as an act of aggression.