The words with prefix plus идти are perfective, while the verbs with ходить are imperfective with the same basic meaning. This is true for all verbs of motion. I suppose that you know that a set of basic verbs of motion are special in that they are either directional (идти, ехать) or not (ходить, ездить). But the moment a prefix gets attached to them they lose that special status and become normal perfective / imperfective verb pairs, with the directional verb being the basis for the perfective verb and the non-directional one for the imperfective one.

There is an odd man out, съездить, which is perfective (there may be more, but that's the one I can think of right now). Furthermore, the perfective verbs using *-ехать have imperfective counterparts using *-езжать and not, as one might think, *-ездить.