Understanding Haste and Overhaste
Most of this information from Samanna's Reference Desk.
Haste is the original Melee Focus, and has been in the game since day 1. Haste basically consists of 3 parts. There are a few exceptions to this, but to put it simply Haste is made up from Worn Haste + Haste Spells/Songs + Overcap (a/k/a Overhaste, a/k/a v3 haste).
Worn Haste and Haste spells/normal bard songs have a haste cap of 100%.
Some bard songs have a higher haste cap, at 125%; if you check Lucy you can identify these higher-cap bardsongs with the denotation v3. Example for Eriki's Psalm of Power "Slot 2: Increase Haste v3 by 30%". All Bardsongs with the v3 stacks with worn haste and haste spells, up to 125%. Chanter and Shaman haste spells are generally called "Haste v1".
For example: You have a worn haste of 45% and have Speed of Salik cast upon you which has 68%. You would hit the first cap of 100%. You also find yourself with a bard who is playing Eriki's Psalm of Power that potentially adds another 30% v3 haste which takes you to the second cap of 125% haste, which currently is the highest haste you can receive.
There are many false ideas on how haste actually works, so let me explain the correct way to interpret haste. If you consider your normal attack speed without any type of haste to be 100% of your attack speed, then full haste makes your attack speed 225% in total. This means you attack 125% more times than if you had no haste. With 125% haste you get 125% more attacks in the same time it took you to make one attack with no haste.
An example: You attack your opponent with a Reaver which has a delay of 40 (attacking every 4 seconds). With 100% haste you land 100% more attacks in the same amount of time, and your effective delay will thus be 20 (attacking every 2 seconds). If you had a bard with you who played the v3 haste song you would end up with a delay of 17.4 (attacking every 1.74 seconds).
There are a few items that add overcap haste (overhaste) like the Symbol of the Overlord from Mata Muram and Silverhoop of Speed from Quarm. These have effects with v3 haste and sources indicate that only the highest v3 haste component takes effect, so they would only be beneficial in a group without a bard (even if the other components stack, like regen etc).
There is also a third type of Haste which is called v2, and are from old Velious era bard songs. They are subject to the 100% cap, and could be used to bring people up to the 100% cap even if they had no 41% haste item at the time. V2 haste is now more or less obsolete, but is still in the game.
There is one more type of haste which is a Monk discipline. This directly lowers the delay of weapons and is unaffected by haste stacking rules.