If you started today LoL launcher then you had to wait to download about 150 MB patch. What's in it from the player perspective? A LOT. Find the whole patch notes here. But after few games we can say that the most visible changes are these:
Many items were changed so you cannot often follow your favourite build. Be ready for it.
Jungle is much more difficult. Minions are giving less experience but are more interesting. Providing living wards, oracle and more.
You can see turtles in the river. These turtles are another ward next to Baron or Dragon after you kill them.
Smite and in general the builds for junglers are useless now. Totally new items are in the shop and you can for example smite the enemy champions (yeah, it's true :-)).
After killing 5 dragons you get very nice burning aura and another bonuses.
Inner towers are providing shields to nearby allied heroes.
Towers are now laser towers. No shots but continuous beam of damage if you are close to them.
There are changes in elixirs. You can have only one active and there are now 4 more expensive ones.
New champion Kalista is available to buy and play.
These are just the most visible changes when you start the game but be sure that the complete impact of them will not be known earlier then in few days or weeks. New builds must be tested, new combos developed, new strategies taken into account...