Here are some basic definitions of English and Pali words you may come across during talks, meditations and study of Buddhist practice.

Buddha - awakening, awakened one, enlightened one, knower.   Also the name Siddhartha Gotama is known as.  

Dharma or Dhamma - the nature of reality or universal truth as taught by the Buddha.  The teachings of the Buddha.  (The difference between Dharma and Dhamma are the Sanskrit and Pali forms, (Dharma is Sanskrit, these are ancient languages that Buddhism was written down in after his lifetime)

Sangha - the community of practitioners, in some traditions it relates only to those who are in robes (ordained) in others it is all who practice.

Mindfulness -  called Sati in the ancient Pali,  is an inward directed gaze, paying attention to the mind, watching it from moment to moment.  The practice of mindfulness allows you to know where the mind is at the present moment, avoiding getting caught up in rumination on the future or past.