Yes Man is an AI program that serves as Benny's assistant in 2281. He is an integral part of Benny's scheme to take control of New Vegas. Yes Man appears to be a modified Securitron. More specifically, his personality program has been put into a PDQ-88b Securitron; although he may later assist Cywren in the "independent" New Vegas options for the game's ending as the Lucky 38's mainframe.
Yes Man was originally a generic Securitron robot programmed to work for Mr. House like all the other Securitrons found on The Strip. He was recovered by Benny and several of the Chairmen after being damaged by a pulse grenade. Following this incident, Followers of the Apocalypsemember Emily Ortal examined the Securitron in order to learn Mr. House's secrets in exchange for reprogramming him for Benny's personal use.
The reprogramming was extensive, and resulted in Yes Man being designed to do exactly as he is told; thus, Yes Man is not just the program's name, it's also its job description. Yes Man is completely incapable of declining any order or withholding any information he is asked for, no matter who he is interacting with. Benny's plan was to dispose of Mr. House and install Yes Man's A.I. onto the Lucky 38 mainframe, putting Yes Man (and by extension, Benny) in control of all the Securitrons on the Strip. The next step was to take the Platinum chip to The Fort, take control of the great weapon he suspected Mr. House of having hidden away (as he did not know exactly what was stored there), and use this "weapon" to assume control of New Vegas and Hoover Dam.