Logging software replaces the paper logbook. You type a call sign, and it records the time, band and mode, looks up the station, warns you of duplicates, and exports the record in a standard format. Choosing one is mostly about which of your activities you care about.
How a logger fits in
Features and names vary by program. Contest loggers lean on speed; general loggers on records and awards.
Information arrives from you, from the radio if the software can talk to it (see CAT control and rig software), from callbook lookups and from spots. The program stores it, uses it to help you in the moment, and exports it to wherever you need it. The standard hand-off is ADIF, a plain-text format that nearly every program reads and writes; see Logging and QSL confirmations. Contest entries use the Cabrillo format.
Kinds of program
Kind
Strengths
Good for
General loggers
long-term records, award tracking, QSL and upload tools
everyday operating, DX, awards
Contest loggers
very fast entry, dupe checking, score and rate display, keyer support
contests
Digital-mode programs
can log contacts as they decode and send
FT8, RTTY and other digital modes
Mobile and field apps
work offline, simple entry, upload later
portable and activation work
Paper
needs no power and no computer
field backup, always-works
Many loggers cover several of these. A contest program that records ADIF is often enough for an operator who does both.
Features that matter
Duplicate checking. Tells you at once if you have already worked a station on this band and mode. In a contest that saves points and time. See Contesting.
Radio control. Frequency, mode and sometimes power are filled in for you, so the log matches what the radio did.
Callbook lookup. Name, location and grid square appear when you type the call; see Callbooks and lookups.
Award tracking. It shows which places or states you still need; see Awards and certificates.
Confirmation uploads. Sending logs to online confirmation systems directly saves doing it by hand.
Import and export. Being able to leave is as important as how it looks today. Test an export before you commit your whole history to a program.
Backups. Keep your log somewhere other than the one machine.