Karma is the belief that the sum of a person's actions in this and previous states of existence, viewed as deciding their fate in future existences; basically, it is an effect, the cause being the actions of a person. In Buddhism, karma is viewed as a moral principal. In Jainism, karma is viewed as a way of moving up or down in status. Being prevalent in both Jainism and Buddhism, karma’s role in Jainism is a way to make life better mentally and spiritually, therefore, is considered more important in Jainism.
First, I’d like to discuss karma in Buddhism and why it has a significance. Buddhism, like Jainism, recognizes inequalities in mankind. Being a religion and a way of life, they feel there is a way to explain these inequalities and that it is not purely accidental; karma being one of those explanations. The other three reason Buddhism gives for these inequalities are heredity, environment, and “nature and nurture.”
Karma in the Pali term means action or doing. Any kind of intentional act is regarded as karma as intentional means there has to be thought put into it. Karma can be either good or bad, depending on the action taken by the person. If the action is unintentional or involuntary, then there is no karma present. The whole idea of karma is that a choice, whether good or bad, is being made, causing a reaction. Being good and making good choices will lead you to be happy while being bad and making bad choices will lead you to be unhappy. ("Basic Buddhism: The Theory of Karma")
Talking a lot about the reaction of Karma, the proper name for this reaction in Buddhism is Vipaka. Vipaka is the result of a voluntary action made by someone. A way to explain the relationship between Karma and Vipaka in Buddhism is seeing Vipaka as a fruit and Karma as the potential seed. When taking care of the seed, you will grow a good fruit; if you neglect to take care of the seed, you will grow a bad fruit. This is where the phrase we often here, “you reap what you sow,” comes from. Happiness and misery are the inevitable effects of causes.
In Jainism, the previous sentence, among many other reasons, are the main differences in the belief in Karma between the two religions. Jainism views Karma as derivatives of karman particles; these particles are both good and bad that become attracted and attach to a person’s soul depending on their actions. ("Jainism Simplified Chapter 8 - Theory of Karma")
Jainism also views Karma as a rewards and punishment system. This differs from Buddhism as Jainism refers to these punishments and rewards through the caste system. The caste system is a social system that determines a person’s status in life; this means you can be a peasant or a king. Jainism says this system and statues are dictated by Karma one gets in their current life. If someone has more bad karma, it is said they will move down in the caste system after reincarnation. If they have more good karma, there is a better chance of them moving up in the caste system after reincarnation. ("Jainism Resource Center - Articles")
Overall, Jainism relies on karma to decide someone’s social status, further dictating their life. People knowing their status due to rewards and punishments and can make decisions to move them within the caste system. Since Buddhism does not rely on karma to determine social status, it is evident that karma is a more important factor in Jainism than Buddhism.

Bibliography
Basic Buddhism: The Theory of Karma. (n.d.). Retrieved August 20, 2015.
Jainism Resource Center - Articles. (n.d.). Retrieved August 20, 2015.
Jainism Simplified Chapter 8 - Theory of Karma. (n.d.). Retrieved August 20, 2015.