I still not really understand Spd in Pokemon means. Basically, in the battle, when my pokemon beat other, the next turn will be the enemy. Therefore, what exactly Spd effect in the battle? it's like the pokemon have higher Spd will go first? is that everything?

Uuuuh, that question seems unrelated to the title...? So I'll try answering both (´・ω・｀)

 

In pokemon, both you and your opponent chooses a move at the same time. Whichever pokemon has the highest speed gets to attack first. This means that with a slow pokemon, you also have to take into consideration what your opponent will do before choosing your move. If there's a tie in speed, it's a 50/50 chance to go first or second.

 

Of course, there's a little more to it. Many moves such as Quick Attack and Protect have higher Priority, while some others like Counter and Roar have lower priority. If both pokemons choose a move with different Priority, Speed is entirely ignored. Speed, in general, does not affect anything else than turn order, but there's also a couple moves, Electro Ball and Gyro Ball, that increase in power depending on how fast/slow you are over your opponent.

 

 

Now EV. This stands for Effort Value, and is represented in blue when looking at your pokemon in-game. Basically, everytime you defeat a pokemon, you gain an EV point in a particular stat. The rule of thumb is that whichever stat is the best for a pokemon, that's going to be the EV he gives when defeated, so a Rattata that only has a good Speed will give Speed EV. EV points max out at 252 for a single stat, and 510 in total.

 

The calculation is that at level 100, each 4 EV points will increase that stat's final score (represented in white) by 1 point. The final score is what is used for all calculations in battle. So, since you're talking about speed, let's give an example with speed. Say there's two identical pokemons facing each other, two level 100 Gengars with the same Nature and IV points (those also affect stats, but cannot be changed after being captured). However, your Gengar is not properly EV trained with only 50 points in Speed, while the opponent is fully EV trained with 252 points in Speed. This is making a significant difference as he has 50 Speed over yours, so his Gengar will play before yours.

 

Hope that answered your questions!