Reggae usually has a 4/4 time signature, and a low tempo.
Reggae can only be played in a 4:4 or 2:4 measure, meaning it has to have 4 beats per bar.
Most of the time there are eighth notes on the high hat. A loud snare or rim shot can almost always be heard on the third beat. The bass drum varies: sometimes it's just on the first beat, the first and the third, every beat, or even just the third.
The standard rhythm guitar in reggae uses quick downstrokes on the offbeat (The 'And' between the beats). 1 and 2 and 3 and 4 and. This is often referred to in reggae as the "skank" and is what gives reggae its characteristic sound. Often you will hear the rhythm being played on the 'and' beats as one quaver beat or two semi quaver beats.
A reggae bass line embodies the "backbone" to the rest of the overall "riddim". Its important to stay simple while occasionally leaving space to coax the rhythm forward. Three things to aim for are denseness, tension, and momentum. Start simple focusing on the root or the 3rd. Inversions are used frequently while using the 5th sparingly adds a certain sense of satisfaction when you finally hear it. Coming in on the strong beats 1 and 3 are helpful while making good use of the 16th note and the triplet is where your groove can be found. Find a motif, use the call and response technique, and play the octave to end a phrase or finalize a musical statement. All the high frequencies (above 1000 Hz) should be filtered out. Bass is often considered the "lead" instrument in reggae because it is responsible (along with the drums) for establishing a song's groove.
The vocals in reggae are often religious or concerning marijuana, but they don't have to be. There are songs about love, bank robberies, and seemingly nothing as well. As it is music "Of the people, for the people, by the people", Reggae lyric often speaks to the masses about raising awareness and vibe, and freeing all people.
Keyboards are mainly responsible for the "bubble", which aids the "skank" to add to the momentum of a given "riddim". Most common reggae keyboards are the Hammond Organ, Rhodes and Clav. They share the rhythm of the guitar usually played in a staccato pattern over the "riddim". The following video demonstrates an organ "bubble/shuffle" - https://www.youtube.com/watch?v=JV0ANC2r6SA
Horns in Reggae are the cherry on top of the "Riddum". Usually utilised to show the difference between "Versions", or simply to deliver after lyric.