As a 121 pilot, I would say there is no real consistency on taxiway naming conventions, in the US anyway. As it was stated, most parallels are a letter, then the related turnoffs are numbered letters (A1, C5, etc).
Generally numbered letters (B2, G2) are shorter, less important taxiways (think side-streets) while lettered taxiways are more like main thoroughfares. But, there are exceptions to this (IAD and DTW immediately come to mind).
Some taxiway systems are named using a location/logic convention, which sounds good on paper, but becomes a disaster when trying to issue/readback instructions over the radio. IAH stands out in this case. There, they have 6 seperate taxiway complexes: North, South, East, West, Ramp, and Far-north. Each taxiway has 2 letters: the first denominates the complex and the second is the name. For example the parallel North taxiways are NB NC and NA while the turnoffs include NJ NN NK NL NE NF. South parallels are SA and SB with turnoffs SH SJ SG. West parallels WA WB WP WC include turnoffs WT WR WM. Ramp parallels RA RB, and Far-north FA FE FB, etc.
IAH tried using logic, but in a pilot's perspective, it is a vocal disaster to have to readback "Taxi to the ramp via foxtrot echo, foxtrot alpha, november echo, november bravo, november foxtrot, cross 26 left"