Red blood cells are red in colour, concave and non nucleated, formed in the bone marrow.White blood cells are colourless

as I see you're in 9th grade, so my answer will be up to your level only.

see there is very important need of 3 things in our body. 1. all the nutrients which releases after digestion of the food which we eat, should must be transported to each and every cell of our body.

2. Oxygen is very important source for breakdown of all those substances which our body cells are getting from the food which we eat. so we have to transport the Oxygen to "Each and Every Cell" so that they can do Breakdown properly.

3. we need a guardian angel who can protect our body from our inner side (i.e. an army to fight with disease causing agents inside our body)

so for all these functions, nature has given us BLOOD.

Blood have basically 4 things RBC+WBC+PLATELETS+NUTRIENTS

now approx 5L of blood flows in our body.

now our 4 problems solved-

1. now all cells can easily take all the nutrients from blood directly.

2. RBC is there for transferring of Oxygen to each and every cell of our body.

3. WBC is our that Gurdian Angel, which protects from the disease causing agents.

so this is a basic level answer, you'll get more detail about their structure and functioning in your respective book.

hope it helps, stay curious!