The Eco-Round ("Economic-Round") most commonly referred to as an Eco or a Save-Round is a money saving strategy that lasts usually one round but may last up to 2 rounds. The Eco-Round consists of buying little to no items. When buying on an Eco-Round it is most common to see a purchase of either grenades or kevlar. The main idea is to save your money for the following round so you and your team can buy together.
Objectives of Terrorist eco-rounds are to either kill as many Counter-Terrorists as you can and best case scenario salvage their guns and/or to get the bomb planted as this yields more money for the next round.
Many of the T-side (Terrorist side) ecos (eco-rounds) are some sort of variation of bum rushes or mass distraction (decoys).
Bum Rushes are exactly what they sound like and try to use the superior number of the attackers to overwhelm fewer defenders usually through the shortest routes of exposure to gunfire/flashes/grenades.
Distraction tactics take advantage of any quick rotations by the other team due to the overwhelming commitment of personnel to attacking one area. The rotation is most effectively achieved by the use of grenades and flashes that results in the best case scenario of yielding at least one kill. The bomb holder then sneaks his way into the bombsite that has been left open or is least guarded and tries for the bomb plant.
One last unique factor of the T-side eco is the low cost of T weapons. This can then result in partial buys consisting of many variations of Deagle, Armor no helmet, Galil, AK-47, SMG, etc. One of the most common eco strats with purchases you will see is Deagle/Armor (the no helmet is implied) where the team depends on the high damage output and maneuverability of the Deagle to quickly bring down a defender while rushing full sprint across areas.
As Counter-Terrorist Eco-Rounds can be successful with stacks on a particular site so the Terrorists cannot take the site closest to the CT stack. If they are not at the location your team stacked then you will have to re-take the site that the terrorist have taken. Objectives of CT-side (Counter-Terrorist side) ecos are to kill Ts and salvage their weapons. Though there is always an unrealistic objective of ninja defusing (hiding from or sneaking past Ts to then defuse the bomb without anybody noticing).
Main strats of CT-side ecos involve stacking and distractions. Stacking involves both personnel and grenades/flashes. Use of smokes to conceal CT movement is common.
Unique to CT-side ecos are exit kills which are attempted after the bomb plant. Because CTs are stacking areas, Ts can slowly determine whether or not certain passages/routes are safe and get the bomb planted. This then forces any remaining CTs to rotate over to the active bombsite. Then either splitting up to cover multiple entrances or stacking one to get kills.
One more thing to note is if you are the last CT alive and the bomb is not planted, it is common to run in Rambo style to hope for a kill. This is done to quickly kill yourself as the bomb plant is something you can then control at that time and stop with your own death.
Mechanics of Eco'ing
The first thing to understand about eco'ing is Counter-Strike's money system. Because of how the game works, the first round (pistol round) of a side is very important. What happens in the pistol round usually affects the next couple rounds. A bomb plant would yield more money collectively so if Ts get the bomb planted then lose the round, there could be sufficient money to partial buy (previously mentioned Deagle/Armor or other combination) even on the very next round (commonly called surprise buy). CTs are not afforded this luxury as often because of the higher cost of their weapons, although surprise buying is not completely unheard of from the CT side.
In higher play, every team member gives a short message of how much money they have either through in-game text or voice to let the leader decide whether or not they are eco'ing or not. This then adds another layer to money management that affects eco'ing and that is the balancing of everybody's budget. Because of the money from kills, not everybody will have the same amount of money even if they are all buying the exact same things. This could then allow somebody who could afford two guns to buy a gun for somebody who has the least amount of money on the team.
Another dynamic is the cost of the AWP. This sniper rifle is one of the most commonly used weapons in the game and is available on both sides, which means the cost is the same to both teams. Eco rounds are sometimes managed to allow the purchase of an AWP by one or more of your team members.