A serial is a story told in a series of consecutively-aired episodes, in which the passage of narrative time is likewise continuous. Each episode, or chapter, ends in a cliffhanger so as to encourage viewers to return the next week to see how the heroes extricate themselves from the previous week's danger.
The 1963 version of Doctor Who was produced exclusively in a serial format, with each serial having between 2 and 14 episodes. Each season had between 4 and 9 serials. Except for the majority of the William Hartnell era, the title of each episode was merely the name of the overall serial, with the episode number appended. Thus, during this era of Doctor Who, the terms "serial" and "story" were synonymous.
The serial format has been largely abandoned by the BBC Wales version of the program, although certain examples, like Dreamland, The Infinite Quest and The End of Time have been produced. Other multi-part stories have been produced, but the production team have been reluctant to return to a numerical naming convention. Likewise, several second parts of stories haven't been set immediately after their first parts, thereby breaking the convention of serial storytelling. The term "serial" is therefore rarely used to describe anything produced by BBC Wales. In current parlance, "episode" is usually synonymous with "story", and multi-part stories are called "two-parters", "three-parters", and the like.