Formal mindfulness practices usually refer to a period of time set aside exclusively for mindfulness meditation. Informal mindfulness practices are any activity that brings us into awareness of the present moment.  The revered teacher Thich Nhat Hahn refers to these as a mindfulness bell.  An example would be to place your awareness on a single respiration cycle every time you hang up a phone, send an email or send a text.  This may give 60 or more opportunities each day to rest in the naturalness of the breath.  This type of informal practice assists the development of formal practice, which in turn deepens the development of awareness.