All right, say your party was exploring some caves. You find a passage back to the surface and as soon as you emerge into the light of day, you are set upon by gnolls. As the party is surprised, the gnolls get a free round which they use to rain arrows down on the party and get ready versus spellcasting. One of the arrows hits your fighter: a sleep arrow. The fighter fails his saving throw and falls asleep. The cleric uses his turn to wake the fighter up. The wizard tries to activate his fireball wand, but doing so triggers the ready versus spell action of two of the gnolls. The gnolls fire their crossbows at the wizard who then fails to cast his fireball. Another round begins, this time the gnolls launch a charge. Two of them position themselves in order to flank the party's fighter, gaining combat bonuses. The fighter attacks and kills a gnoll outright. He has cleave and gets a free attack on another.
These are typical actions you will take in combat. Your characters and their opponents have a lot of options to choose from, including five foot step, charge, bull rush, grapple, pin, casting a spell, ready versus approach, ready versus spell, delay, counterspell, using the inventory, whirlwind attack, breath weapon, etc. Ready versus spell is maybe the most interesting. An enemy wizard may use it against your wizard, while your wizard could use it against another opponent; and if that opponent casts a spell it will trigger a chain reaction.