Tantra in its most simple terms is learning to expand awareness of and work with life force energy as a way of exploring creational forces, life, Spirit and the individual practitioner's relationship to all these things. The Tantric goal is to use these practices and tools to discover the True Essential Nature of the Self and all of life as being Sacred and Spiritual in Nature. Or in Tantric terms: Everything, including the Self, is Pure Consciousness.
Tantra offers a huge variety of different tools and techniques to help guide the practitioner towards this end. The common thread running through these practices is that of increasing life force energy within the body and mind while cultivating focus and awareness. This results in "purification" of the mind and energy systems and allows the practitioner to perceive increasingly subtle layers of energetic experience. As one moves through the subtle layers of experience, one necessarily must explore the increasingly subtle layers of body, emotions, mind and spirit.
Healing versus Spiritual Advancement
Reiki as a healing system teaches us about our energetic nature, the spiritual essence of our Being, and teaches us to work with the Reiki energy as a means for balancing the body, mind and spirit. If I shift the intention of Reiki from "healing system" to "spiritual advancement system" then I am now working to increase life force energy and explore these subtle layers as a means for spiritual awakening, the very same exploration one needs as a tool in Tantra.
The main difference between "healing system" and "spiritual advancement system" is that of intention and desired outcome. A "healing system" is primarily focused on the relief of symptoms: reduce pain, heal illness, balance emotions, calm the mind, etc. A "spiritual advancement system" is concerned with deepening awareness of the spiritual nature of reality and the Self.