Alex, K0UOG
As a ham radio operator, I enjoy tracking the contacts I've made on the bands, and I'm certainly not alone. Even though there's no legal requirement to log your radio activity—the FCC eliminated logging requirements in the amateur service in November 1982—most hams still maintain logbooks, and many reasons still remain for keeping a detailed log. Most contests, special events, and award sponsors require logs for maintaining, tracking, and scoring their events. Many hams track space weather and band conditions in their logs to plot the performance of their equipment and configuration over time in varying conditions. Good log data could be a deciding factor in your favor if a complaint of improper operation or interference were brought against your station. Whatever the motivation for logging, ham radio operators around the world record thousands of contacts every year.
The hardcopy paper log, which the ARRL calls "the traditional keeper of the contacts," is still in use, but with the advent of computer logbooks, it is becoming more rare. One place where paper logs are still relevant and useful is in the field, even though in many instances the field logs ultimately are destined for a digital logbook. As a Parks on the Air participant, I use paper and pencil to log my contacts while operating portable in the park, but when I return home, I enter the data into a computer program for my personal recordkeeping and for submission to POTA. The Amateur Radio Emergency Services (ARES) often rely on paper forms—particularly form ICS-214—for logging of radio transmissions and other activity during an event or activation, and while the ICS-214 is not amateur radio-specific and serves a vastly different purpose than regular contact logging, the information captured on such forms can later be merged into an existing contact log as supplemental data.
Not surprisingly, in the early days of computer logbooks for amateur radio, methods of storing and formatting log data proliferated, perhaps not least because of the do-it-yourself nature of many amateur radio enthusiasts who were eager to learn computer technology and extend its capabilities into the amateur radio world. One of the main drivers of computer logging advances in ham radio were contests. Scoring contests using paper logs was time and resource intensive, requiring participants to wait weeks or even months before receiving results. One of the early standards for exchanging contest data electronically was Cabrillo, and it is still in widespread use today. The World Wide Radio Operators Foundation maintains the Cabrillo standard, but a number of contest operators have customized the specification for their own contests. As a result, logging software often includes a myriad of Cabrillo templates by default. Despite proliferating variants of the format, Cabrillo remains the gold standard for contest logging, where the on-air exchange is often brief and uniform within any given contest.
If Cabrillo was designed for contest logging, what about day-to-day station logs? The Amateur Data Interchange Format (ADIF) standard is the most widely used general-purpose ham radio logging format. Originally created in 1996 and developed over the years, ADIF allows the logging of almost any conceivable aspect of an on-air exchange, ranging from the routine (callsign, band, mode, frequency, location) to the more detailed (solar flux index, K-index, A-index, mailing address, QSL status). ADIF allows the tracking of contacts and station performance through time based on radio equipment, antenna, band conditions, and location.
It's important to note that ADIF and Cabrillo each serve a specific need and can easily co-exist in your shack. Most modern computer logbooks can export and import data in both formats, and there are many programs capable of converting between the two formats. Sometimes ADIF and Cabrillo are erroneously equated with the software programs that read and write data in these formats. These standards provide a way to transfer ham radio contact data between disparate software programs and systems, serving a purpose similar to HTML on the web. In that analogy, the various ham radio logbook programs (Ham Radio Deluxe, ProLog, etc) are the web browsers (e.g. Chrome, Firefox, Edge), and ADIF and Cabrillo are the software-agnostic HTML format incorporated into all the web browsers.
If you are participating in contests or special events (POTA, IOTA, SOTA, etc) in ham radio, you will need to be at least somewhat familiar with ADIF and Cabrillo. The web has a plethora of information about these standards—much more than I can fit into this short-form article. I've included a few links below with relevant information.