There are already two answers posted with good information but here is a little bit more information to help with your selection process.
Matching the impedance of the speaker to the speaker impedance range specified for the audio amplifier (the transceiver in your case) is important in order to ensure a maximum power transfer from your radio to your ear. An improper impedance can also introduce distortion and potentially instability in a marginally designed amplifier.
Most speakers have a Sound Pressure Level (SPL) rating. This is an indication of how efficiently the speaker converts electrical power to acoustic power. It specifically is the sound pressure level measured at 1 meter from the speaker with 1 watt of excitation applied. The result is generally expressed in dB form. In general for a common speaker design, the larger the speaker, the greater the SPL.
A higher SPL indicates a more efficient speaker. This can be important for hearing the subtleties within the sound source as a lower SPL can render low level sounds as imperceptible.
The frequency range of the speaker indicates what frequencies the speaker will reasonably reproduce. This area of characterization lacks standardization so often a close inspection of data sheets are in order to make comparisons. The general bass, mid range, and tweeter components used in audiophile systems are examples of speakers that are optimized for specific frequency ranges. The frequency range required for a specific amateur radio application depends upon the modulation mode. An standard SSB voice will be faithfully reproduced in a 200 to 2500 Hertz range. A CW signal can be used with a much narrower frequency range signal based on the preferred side tone frequency of the operator. An FM signal often has a wider response than standard SSB. Some deficiencies in frequency response can be overcome with a parametric equalizer added to the amplifier chain.
It should also be noted that a speaker with too large of a frequency range may prove annoying as it will be more prone to reproduce noise that is accompanying the desired signal.
The speaker enclosure can have a dramatic effect on the frequency response and in some cases, the effective SPL of the speaker. A common example of this is acoustic dampening within the enclosure. Wool will generally modify the mid range frequencies while felt will modify the low end frequencies. Other methods effectively alter the geometry of the enclosure for certain frequencies. Examples of this includes porting and acoustic foam. You will find a great deal of subjective and objective information on this topic on various audiophile sites.
From an amateur radio perspective, the enclosure may also play a role in keeping RFI out of the amplifier chain.