7th chords (aka Dominant 7th chords)
Any number at the end on a chord means there is an extra note added. This doesn't always make it harder to play, in fact it can make it much easier! 7th chords always sound 'bluesy', just like how minor chords sound 'sad'. These dominant 7th chords are great for getting an authentic blues sound, and can also preempt a change in a chord sequence, like in Hey Jude.