From: http://www.lamazopa.com/onkarma.htm

There are 3 types karma:

karma created in this life which results are experienced in this life

karma created in this life which results are experienced in the next life

karma created in this life and which results are experienced after many lifetimes

Karma that is repeatedly done becomes very powerful. Karma that is done in relation to a powerful object such as one’s parents, etc. also becomes very powerful. This is true of even small harmful or disrespectful actions on the one hand, or small actions of service and respect on the other. This is especially true of actions done in relation to the guru, Buddha, Dharma, and Sangha. These are very powerful objects. Consequently, one can experience in this life the suffering results of negative actions committed in relation to the guru, Buddha, Dharma, and Sangha, as well as experience in this life the favorable results of any small virtuous actions done in relation to the objects of refuge.

The four general outlines of karma are:

1) Karma is definite. This means that having committed an action, one will definitely experience the appropriate result according to the kind of action done: virtuous actions bring positive results and non-virtuous actions bring suffering results.

2) Karma is expandable. This means that once karma is created, its results will be experienced in many, many future lifetimes. Thus, the suffering result of a non-virtuous action will be experienced many times, not only in this life but in hundreds or even thousands of lifetimes to come. It is said by Nagarjuna - the great pandit who is like a second Buddha and who propagated the Mahayana teachings, especially those on emptiness - that if one cheats one sentient being, then the result will be that one will be cheated for a thousand lifetimes by others. This is from Nagarjuna’s text Letter to a King. One shouldn’t be careless even in doing one small negative karma thinking this wont hurt me.

It is mentioned by Buddha in the sutra Dul lung, one small fire spark can burn piles of hay mountains high. Same, one shouldn’t think one small virtuous will not benefit me. Like the example, by collecting drops of water one a large bucket can be filled! Therefore one need to, by practicing remembrance and awareness, one should abandon even the smallest negative karma and practice even the smallest virtue.

3) Karma always has a cause. If one hasn’t created the karma, that is, if one has not done the action that is the cause, one can never experience the result of that.

4) Karma created is never lost. This means that once karma is created, it will not disappear of its own accord. If virtuous or non-virtuous karma is not interrupted, then it will never get lost. However, if karma is interrupted, then it can be destroyed. For example, by purifying a non-virtuous action, you can interrupt the potential to experience the suffering result. In order to interrupt negative karma, one must purify it through practices such as strong confession. If one has created negative karma such as the ten non-virtues or other negative actions, just living one’s life being scared, upset, and depressed, that alone is not a solution. The solution is to do something to purify the negative karma immediately. If negative karma has been created, then it can be purified.