You tend to have two types of processing done in a mainframe, batch and online. Transactions usually refer to online work. In the old times, it used to be someone connecting in 3270 and issuing a bunch of "transactions" to bring up screens and perform various operations. Now, it would likely involve a web layer. The server part can be CICS for example, but they are others. Think of them as application servers. Proper batch jobs tend to perform even faster. There is a few reasons to that, one of them is the full integration of the systems (ie: the batch has "shared" memory access to the database, basically it's a big stored procedure).