Spamming is the act of excessively using the same tactic over and over or building an excess number of the same unit in the hopes of overwhelming or surprising the opposing player. While this strategy (if it even deserves that title) may overwhelm an unwary opponent, it is ineffective against someone who is well established. Some players would create massive forces made up of just one type of unit (i.e. tanks, fighters, bombers) and then send them on a virtually irresistable wave against the other player(s). However, in R.U.S.E., due to the ability of players to see the moves of their enemy, to some extent, spamming is easily countered. For example, many AA guns are cheap and quick to build, eliminating the threat of a bomber/fighter spam. Mobile AA can rip apart an air attack as well. Britain's Archer Tank Destroyer is relatively cheap, and can eliminate threat of tank spam. Italy's Semovente Tank Destroyer, while not as cheap, is even more effective while still maintaining a good price.
Many players like to gather masses of artillery in an attack, but this usually does not count as spamming, as the artillery is generally protected or supported by other types of units; it would be easily destroyed otherwise by air attack or flanking.
One type of common spam which actually works is the Italian tank rush, in which masses of Italian medium tanks, usually Carro M13s and M15s, are sent in a relatively early game period of a match and can catch other players off guard. However, Italian units are poor (the tradeoff for being cheap) and can be destroyed more easily, so artillery and air units can break up this type of mass attack
Spamming and Rushing are two different tactics and should not be confused. Rushing involves the strategic use of similar units, such as Light, Medium, and Heavy Tanks, to overwhelm the enemy and usually takes place during the early game. Spamming is much more reckless because it involves spending most resources on one specific unit or building type.
Building large amounts of high quality tanks (such as King Tigers) can be a great attack/defense method. You need to cover with AA and fighters, though, to make sure you aren't destroyed by air power. Keep recon around so you can see incoming attacks.
This tactic is generally frowned upon by many players of R.U.S.E.; although, this should only be overlooked as the same players that would scold you for spamming would just repeat the same offense.