Walk around using the arrow keys. Click a Pokemon to engage in battle (if you're close enough). The Pokeball will follow your mouse. Click to throw. If no Pokemon spawn, press space. "Team" shows which Pokemon you have caught.
UPDATES 07/01/18 - Minor fixes and more Pokemon 03/10/16 - More Pokemon 29/09/16 - Day&Night system, new pokeball animation 27/09/16 - Capture rate 25/09/16 - Shared Pokestop and Gym (not used in-game) sprite from @Pokedrew2 Pokemon, trainer and pokeball sprites from Pokemon Gold. Music from Pokemon Go. Pokemon belongs to Nintendo.