Any thoughts, speech or actions that are rooted in greed, hatred and delusion and thus lead us away from Nirvana are bad and any thoughts, speech or actions that are rooted in giving, love and wisdom and thus help clear the way to Nirvana are good.
To know what is right and wrong in god-centred religions, all that is needed is to do as you are told. But in a man-centred religion like Buddhism, to know what is right and wrong, you have to develop a deep self-awareness and self understanding. And ethics based on understanding are always stronger than those that are a response to a command.
So to know what is right and wrong, the Buddhist looks at three things – the intention, the effect the act will have upon oneself and the effect it will upon others. If the intention is good (rooted in giving, loving and wisdom), if it helps myself (helps me to be more giving, more loving and wiser), then my deeds and actions are wholesome, good and moral. Of course, there are many variations of this. Sometimes I act with the best of intentions but they may not benefit either myself or others. Sometimes my intentions are far from good, but my actions helps others nonetheless. Sometimes I act out of good intentions and my acts help me but perhaps cause some distress to others. In such cases, my actions are mixed – a mixture of good and not-so-good. When intentions are bad and the action helps neither myself nor others, such an action is bad. And when my intention is good and my action benefits both myself and others, then the deed is wholly good.