While many songs may have the same chord progressions and follow similar scales, how they are played can make all the difference. For example the time signature alone can be a defining aspect of the song itself. Take the theme from Mission Impossible, it has a somewhat driving sound based upon its 5/4 time signature, that extra beat per measure gives it a sense of urgency. The song Money by Pink Floyd has a unique feel based upon a time signature of 7/4, and of course we get the entire genres of waltz and marching music all in 3/4 and 2/4 time respectively. These different time signatures can give a song a particular sound, and even though most modern music is in 4/4 time, it depends on how that 4/4 time is played that can make all the difference.
We can take a 3 chord song, a common progression like G-C-D, and depending on how we strum and the rhythm we give it, the song can be one of multiple genres. In Rock n Roll the beat is often emphasized on beats the 2 and 4... one TWO three FOUR... playing it in that fashion will give us something upbeat and rocking. In reggae the guitarist often plays on the offbeat... one AND A two AND A three AND A four AND A... giving us a laid back vibe completely different than hard rock. So even a song that has the same time signature and chord progression can be played in so many ways that each one will sound like its own unique song, with the progression G-C-D we can play most major genres of music. This interesting video lesson shows you how to strum chords in the right way, check it out.
The notes that are played per measure can establish what style we are playing in from the very beginning of the song. A simple quarter note down strum can give us a folk song or a simple slow ballad. If we want to spice it up a little bit we can add in eighth notes, often the eighth note is played as an upward stroke. So if we play 3 quarter notes and 2 eighth notes we get down, down, down, down, up, which is a perfect strum for a catchy country ditty. Using the G chord try that strum out and you will hear that country/folk sound. If we use all eighth notes in one measure, with all downward strokes we get a very common blues shuffle... one and two and three and four and. And then of course we can add 16th notes into the mix, this is commonly used in funk music. The rhythm guitarist for funk plays a series of up and downstrokes very quickly giving that signature funky sound. For a punk band the guitarist will be playing a lot of power chords with downstrokes.
There are honestly a huge amount of guitar strums in existence, and you will find that even when you play common ones that there will be a tendency to shake things up a little bit. Rarely does a guitarist play a strum the exact same way through the whole song. The key to understanding different strums is to pay attention while learning a new composition and try and copy the sound with your hand. It's never going to be an exact science of up, down, up, down and so on, certain strums require a bit of feel and finesse and it is your job to use your ears to copy what you hear. Music has a particular science to it and each genre has specific rules, but it is up to the musician to take that science and those rules and add some flair and creativity. When it comes to strums pay close attention on how to play it, but eventually let your creativity take over. Always use strict practice when learning a new technique and then later add unique changes as you progress.
Allowing your creativity to take over will lead to syncopation in your guitar rhythm and strums. Syncopation is the changing of the expected rhythm, instead of playing all quarter notes we unexpectedly add some eighth notes in and mix it up a little. We can also mix it up by holding certain notes longer than others, sort of fudging the numbers a little bit. It is often difficult for programmed drum machines and computers to get a really good shuffle and swing feel to them. That is because it takes a human touch on extending certain notes and changing up the strumming pattern to give us a different feel in the music. Here's a video lesson with a creative approach to strumming.
Strums can also be dependent on who else is playing along with you, if anyone at all. If you are in a band as a rhythm guitarist than you can simply stick to playing a chord progression in a designated strum, while the other band members do their part. If you do not have a bass player, than as a guitarist you will want to add some bass notes in before and after certain chords. If you are alone without a drummer you may want to emphasize certain beats to help the listener get a feel for the tempo and beat. To put it simply there is just no easy definition of how to strum a guitar. Of course there are a multitude of books and lessons that will teach specific strums and those are great and fine to learn. However after learning a few you will find you start to inadvertently mix strums or change quarter notes to eighth and so on.
One of the best ways to learn strumming and rhythm is to listen to some of your favorite music and then attempt to play it on your guitar. Participate in active listening, use headphones, repeat the song, listen for downstrokes and upstrokes, listen for note length, and focus on any emphasized beats. One of my favorite genres is reggae, and my initial entry into it consisted of trying to get that same offbeat guitar sound. I kept playing until I had the right chords and the right strum to the song I was listening to. At the end of the day, strumming a guitar is about emotion, let that feeling that the music gives you move into your picking hand and just practice until it sounds great!