Rakhi is a sacred thread.
According to Hindu scriptures Rakhi can be defined as a thread made of cotton thread that can be tied on hand with pure wish. It can be even a piece of your cloth!!
It is kind of bracelet. Sachi tied a bracelet on Indra's hand.
Any design is accepted. Because the wish for good has only one design -> purity.
Sisters buy cartoon rakhis for brothers who are children. These brother will love cartoons , so they buy it. Reasons may vary for choosing a rakhi. Mostly colors are golden or red, but one may choose other colors.
Some brothers like Rudraksha Rakhis.
But the importance is sister's prayer for his brothers' well beings. These prayers have power. That only make Rakhi protects his brother.
Click Here, there are many incidents from Hindu scripture based on Rakshabandhan.
Hindu scripture has defined a rakhi to be sacred thread only. Whenever the term Rakhi is used in texts, in most texts it is referred as cotton thread or bracelet.
Meaning of the rakhi
The word Raksha means protection, whilst Bandhan is the verb to tie.
It is believed that when a woman ties a rakhi around the hand of a man it becomes obligatory for him to honour his religious duty and protect her.
Traditional stories state that rakhis are blessed with sacred verses and are encompassed by them.
Sometimes rakhis are consecrated in rice and grass before they are given, and they are traditionally tied by people familiar with the Vedas.
Following these customs the rakhi is believed to remove sin from one hand and provide safety to the other. The protection offered by a rakhi is believed to remain for a year.
Ref - Rakshabandhan