Being a reseller just means you sell domain names for them, they got the ICANN accreditation and the technology, you do the marketing and selling.
Pros-
-cheaper registration for yourself
-you get a cut of all sales (you set prices within a realistic min./max. range)
-relatively cheap and easy to get started
-GD provide all the technology and accept the creditcard/paypal payments for you
-GD handle customer service (unless you choose them not to)
-you can actually undercut GoDaddy without coupons (I just use my own reseller account for $6.95 .coms without looking for a GD coupon)
Cons-
-GD charge an annual fee to be a reseller (although 'Super Resellers' are able to offer the reseller packages at a discount- see my
dnReseller.biz GD's $99.99 reseller is only $60pa)
-Ugly interface (much like the GD homepage)
-Difficult to customise