While using aid another successfully on a skill checks grants a circumstance bonus, the name of the bonus granted by the special attack aid another is unclear… yet in both cases it likely doesn't matter
The aid another concept comes in two flavors.
First, one creature can aid another creature on a skill check. The Player's Handbook in the chapter Skills on Aid Another says, "If you roll a 10 or higher on your check, the character you are helping gets a +2 bonus to his or her check, as per the rule for favorable conditions" (66–7). Favorable and Unfavorable Conditions says that one of the ways that the "DM can alter the chance of success [is] to take into account exceptional circumstances [by giving] the skill user a +2 circumstance bonus to represent conditions that improve performance" (64). Thus, when aid another is used to improve a skill check, the bonus aid another grants is a circumstance bonus.
I have no idea why that Aid Another section omits naming the bonus from aid another a circumstance bonus, instead making the cautious reader flip back three pages to check that earlier section. However, a reader that plows ahead in that Aid Another section anyway learns from the example that the bonus is a circumstance bonus: "If the other character makes a Heal check against DC 10, then Jozan gets a +2 circumstance bonus on the Heal check he makes to help Krusk" (66 and, like most examples, absent from the SRD).
Similarly, the Rules Compendium on Aid Another doesn't mention that it's discussing circumstance bonuses until Expert Assistance, its final subsection on the topic:
If you have 5 or more ranks in a skill you’re using to aid another, you can grant a higher bonus. For every 10 points your check result exceeds 10, the circumstance bonus increases by 1. To determine the circumstance bonus quickly, simply divide the helper’s check result by 10, round down, and add 1. (ibid.)
(Emphsis mine.) The Rules Compendium, by the way, cribs here from Complete Adventurer, and on Aid Another in that earlier text the bonus from aid another is consistently named a circumstance bonus (96).
Second, one creature can take a standard action to make the special attack aid another to help the first with its next attack or against the next attack made at it. The Player's Handbook's chapter Combat on Aid Another is even less clear that the special attack aid another grants a circumstance bonus, but it does say, "Multiple characters can aid the same friend, and similar bonuses stack" (154). The Rules Compendium is similarly tight-lipped about what kind of bonus this is, but Complete Adventurer says that expert assistance with attack rolls grants bonuses "as described above," which are only named circumstance bonuses.
Fortunately, for both pedants like me in the first case and the one Medium creature and his 7 to 23 Medium allies (some of whom have reach weapons) that are aiding him against one Medium foe in the second case, the name of the bonus from either flavor of aid another doesn't really matter: if the bonuses are circumstance bonuses or if the bonuses are unnamed bonuses, if each is from a different source, then all of them will stack.
That is, the Dungeon Master's Guide explains, "Circumstance modifiers stack with each other, unless they arise from essentially the same circumstance" (21)—and different helpers apparently are different circumstances (see PH 66)—, and the Rules Compendium straight-up says, "Untyped bonuses stack unless the bonuses come from the same effect" (21).
Note: This writer prefers the term unnamed bonus because unnamed is recognized by his real-time spellchecker and untyped isn't. No need to worry, though; they're the same thing.