The Czechs and Slovaks share a very similar history, aside from the fact their languages are incredibly similar. Most Czechs, especially in the Moravian part of the country can speak and understand Slovak, and vice-versa.

Both Czechs and Slovaks were grouped together in the same "country" for a couple of hundred years between the Habsburg Monarchy, and the Austro-Hungarian Empire. Hungary was part of that of that too, but the Slovaks were close in history to the of the Czechs.

Fast forward to the end of WW1 and dissolution of the Austro-Hungarian empire. Austria went its separate ways as did Hungary, etc. the Czech and Slovaks were content on sharing a political entity that would become Czechoslovakia. They had many different political and social goals at the outset and led to them sharing a border together

Im on mobile and can go a lot more in-depth tomorrow when Im at my computer, but I studied Czech history for my undergrad, and doing Czech studies for my Graduate program :)