CompTIA (which stands for Computer Technology Industry Association) has evolved into an non-profit organization responsible for accrediting Information Technology industry professionals (from its roots as the former Association of Better Computer Dealers, Inc.)
To be accredited under the CompTIA system, you have to take the exams administered by the Prometric, and the Pearson VUE, testing organizations. A CompTIA Voucher is a sort of ticket showing you are authorized to take the IT accreditation exams to qualify for CompTIA accreditation.
If you opt to apply with Pearson VUE, you have the choice between paper vouchers or electronic vouchers. A paper voucher is convenient to have along if you prefer to present it along with other hard copy documents. An electronic voucher is a roster of voucher numbers, rather than a hard copy document. Either way, you can present either to be able to take the exams.
You need to use all of the vouchers you have applied for before their expiration date, because once they are expired, they may no longer be used and you cannot have them refunded. Your CompTIA voucher is only valid for up to 12 months from date of purchase after that they will be considered expired.
If you buy a Pearson VUE voucher, you may only use their vouchers at their Pearson VUE testing centers.
A Prometric voucher functions just like a Pearson VUE voucher, but you may only use it at a Prometric testing center.
The advantage with using either a Pearson VUE or Prometric voucher is that you save money because these exam prerequisites are offered at a lower price or discounted price (compared to the price paid by those who do not use exam vouchers at all.)
If you come from countries outside the US, you need to check if the vouchers you purchased are valid in the country where you plan to take the CompTIA exams.