идти, ехать - imperfective progressive
to be in the process of one-directional motion
ходить, ездить - imperfective iterative
to perform repetitive or multidirectional motion
progressive:
Я иду по городу. I am going along the city (right now and towards some destination).
Я иду в магазин. I am going to the shop (right now).
iterative:
Я хожу по городу. I am walking around the city (right now but without one destination, multidirectional)
Я хожу в магазин. I go to the shop (Usually or from time to time, repetitive)