Internet Merchant services
Internet Merchant services are popular for a number of reasons and they are easy to use. With a Internet merchant account you can send and receive money over the internet fast. 

If you are betting online you will probably want a Internet Merchant Service. If you are having payouts wired to your Bank Account or you do not mind waiting for a Cashiers Check, Check, or Money Order then a Internet merchant account is not entirely necessary (but still highly recomended).

Depositing funds to an account(s)
The most common ways to deposit are Bank wire, Visa, Mastercard, and money transfer through online Internet Merchant Services (listed below).

!!WAIT Before you select a Internet Merchant service to use to deposit funds you should check at the place where you plan to play first.  Different companies use specific Internet Merchant Services and oftentimes they are not the same. You have to use a Internet Merchant service the gaming site uses.  Another important reason to check the gaming site first is sometimes the gaming site will give you a bonus just for signing up to a particular Internet Merchant Service.

*Note: While many people use either Visa or Mastercard to deposit funds, using Visa or Mastercard to deposit funds usually works fine but it is becoming a thing of the past. 

a) Some banks have stopped allowing deposits to betting sites. 
b) Neither Visa or Mastercard can be used to withdraw payouts.