The Rambam (Sefer haMitzvos 94) lumps all forms of nedarim – whether it be a simple personal pledge or a neder to being a korban or give tzedaka - together under one mitzvas aseh of ‘motza sefasecha tishmor’. The Ramban, however, disagrees, and divides nedarim into two distinct categories: nedarim that are personal in nature, ‘nidrei reshus’ or ‘nidrei bituei’, which we are commanded in parshas matos to fulfill, and ‘nidrei mitzvah’ or ‘nidrei gavoha’, which is the mitzvah of ‘motza sefasecha tishmor’. Ramban in S”haM writes that nidrei gavoha are binding even if one does not use the term 'neder', e.g. if one says ‘beheima zu korban’, one is obligated to bring the animal as a korban; however, nidrei reshus are binding only if one uses the formula of neder (or some equivalent of a yad or kinuy), e.g. ‘konam ochel zeh alai’ - just saying ‘hareini nosein beheima l’ploni’ is not a valid neder. Another nafka mina between these two categories is with respect to the issur of bal t’acher – nidrei hekdesh must be fulfilled within the time span of 3 regalim or one violates an issur; this requirement does not apply to nidrei bituei.
Ramban reiterates his position in the opening to Parshas Matos. Rashi (30:3) gives an example of a neder as ‘hareini oseh davar ploni’, I will do action X. Ramban writes that no where in masechet Nesarim do we ever find a case of nidrei bituei b’kum v’aseh, nedarim which require one to take action. A neder by definition is an issur cheftza, a prohibition on an object, and therefore must relate to something concrete, not actions. The Ramban challenges his own thesis: don’t we find cases of nedarim which demand action, e.g. ‘beheima zu yehei korban’, I will bring this animal as a korban? The Ramban answers l’shitaso that this example is from the realm of nidrei gavoha, which is a different domain than nidrei bituei. Since amiraso l’gavoha k’mesiraso l’hedyot, pledging a korban creates a kinyan to hekdesh, it is as if the animal in question has already been transferred to the ownership of hekdesh by virtue of the verbal neder and therefore the pledge is binding. (See Ramban for a second reason why such a neder works.) It seems from the Ramban that the definition of nidrei gavoha are nedarim which effect not just a prohibition, but which have an immediate financial impact, a dinei mamomos component. Homework: do all cases of nidrei gavoha indeed fit that mold?