Diskspace, in terms of reseller accounts, has to do with the amount of disk space that you are given to share between all of your accounts (that includes your primary account and any resold accounts). You can divide this diskspace up to best suit your reseller hosting needs from within the WHM control panel.
Your bandwidth is the amount of data you can transfer both to and from your reseller hosting plan. With the reseller hosting accounts, the bandwidth limit is shared between all of your created (and primary) accounts. You can easily set your own bandwidth limits for each resold account through the WHM control panel.