Accuracy - of course refers to how close to the time set the actual time measured is.  The figure is expressed as a plus/minus figure indicating how much the time may vary from absolute accuracy.  It will most often be shown as a percentage, but sometimes is shown in fractions of a second.  Obviously, the lower the figure the better.  Generally, mechanical timers are the least accurate, while electronic timers can be accurate to the extreme.  While accuracy is desirable, it's not as important as "repeatability."  For example, if you set the timer for 10 seconds and the actual timing cycle is 11 seconds, that's not terribly significant if you know that setting the timer to 10 seconds delivers the proper exposure.  

Analog - refers to timers with sweep hands like a clock or watch, such as the Audible Repeating Timer and Pro Lab Timer.

Audible signal - most users expect the timer to indicate the end of the exposure or process step with some sort of beep or other audio signal.  This is more important for process timing, since the user often is occupied with something else while waiting for the step to finish.  Most timers do provide this, but it can be advantageous if the signal can be switched off and/or the volume adjusted.  See also Metronome.

Digital Timers - with LED (light emitting diode) displays are electronic timers based on an IC (integrated circuit) chip.  They have become extremely popular as they are easy to read and provide a visual countdown of the remaining time, much like electromechanical timers with sweep hands, but as with most electronic timers, with greater accuracy.  Since LEDs are usually red in color, little correction has to be made to prevent paper fogging, although the brightness is not usually great enough in most circumstances to cause a problem.  Some units may provide a control to adjust the brightness.  LCDs (liquid crystal display) are rarely used because they do not emit any light and therefore require a separate means of illuminating the display.

Electromechanical Timers - have an electric motor to power the clock mechanism, but are still largely based on mechanical gears and switches.  The Audible Repeating Timer and Pro Lab Timer are examples of electromechanical timers. 

Electronic Timers - are completely based on electrical components and have few if any moving parts.  Early electronic timers used mechanical rotary and toggle switches, but now many timers have only a keypad for setting and control purposes.

Enlarging timer - Since exposure times are relatively brief compared to processing steps, most darkroom workers prefer to have a separate timer for enlarging that measures only seconds, and, in many cases, fractions of seconds.  The majority of Omega timers are enlarging timers.

Footswitch - a convenient accessory for an enlarging timer, since control of the timer and thus enlarger can be made by a tap of the foot, leaving the hands free.  Some footswitches have a single pedal, which acts as a start switch, while others have dual pedals, with one pedal turning the enlarger on and off while composing and focusing, and the other to start the timing sequence. The dual pedal arrangement is therefore the most convenient, so note which type is available for the timers you're considering.

Load - all timers that have electrical outlets for the enlarger and safelight or accessories, will have the electrical capacity of those receptacles marked with a maximum figure, expressed in Watts, to ensure there will be no electrical hazard.  Most timers will be able to safely handle the electrical load of the enlargers and safelights used in a home darkroom, but it's wise to check the timer's capacity before purchase.  You may also find that some electronic timers have the maximum load expressed as two figures - the higher one for enlargers with power supplies, and the lower figure for enlargers with tungsten enlarging lamps.

Mechanical Timers - years ago, virtually all darkroom timers were based on a spring powered mechanism.  That meant that the enlarger had to be turned on and off manually while watching the timer - not very accurate.  The Omega 60 Second mechanical timer improved on this by incorporating a switch to control the enlarger.  However, since the circuitry for electronic timers can be produced so inexpensively now, and deliver much better accuracy and repeatability, there are few mechanical timers left on the market.

Metronome - During an exposure, it's often helpful to be able to hear the timer counting down the seconds, particularly when doing dodging or burning in when you're unable to look at the timer.  Some timers therefore have an audible signal at each second.  Since this can be annoying when not needed, it's a definite plus that there is provision to turn it off when not required.  Still better is a volume control so that the loudness can be adjusted to suit the individual.

Process timers - are required to measure minutes and seconds.  The most common process timers will require the user to reset the timer for each step.  The Pro Lab timer, for example, can be used in this way.  However, some timers are capable of being pre-programmed for all the steps in a particular process, and all that is required is to restart the timer for the next step.  Although they differ greatly in construction and the way they are used, both the Program Timer and CT-40 are examples of programmable timers.  If there is an electrically powered device that must be started and stopped during processing, such as a color agitator, it is advantageous for the timer to have control of the accessory.  Most process timers will have an outlet into which the device can be plugged.

Repeatability - this term refers to how close to one timed cycle the next ones at the same setting will be.  It's actually more important than "accuracy."  If you have determined that 10 seconds gives you the exact exposure required, you want the next exposure at 10 seconds to be identical, and not vary significantly one way or the other.  Like "accuracy," you'll see this figure expressed most often as a percentage, and the lower the figure the better.  

Safelight switching - when composing or focusing, it is often advantageous to have the safelight off in order to provide the greatest clarity of the image on the easel.  Most enlarging timers have a receptacle for the safelight and normally this outlet is switched off when the timer is in the "Focus" mode or actually timing the exposure.  Some inexpensive timers do not provide a safelight receptacle.  A few of the more deluxe timers have a switch to allow disabling of the auto switching feature.

Two Dial type electronic timers - refers to electronic enlarging timers which use two mechanical rotary switches to set the time, but which do not have any display for the countdown.  They can usually be set for  0.1 to 99 seconds in two ranges - 0.1 to 9.9 sec., and 1 to 99 sec.  On most 2 dial type timers, a toggle type switch is used to change between ranges.  The Omega Precision Timers had an unusual variation - the right hand dial when turned to the left had 0.2 second increments, but turned to the right had 10 second increments.