God is known as Jehovah Rapha because He is the ultimate healer.  However, it doesn't only just mean that He heals disease.  While some people would say it is a part of God's name, His nature is one.  We don't pray to Jehovah Jireh as a separate part than Jehovah Rapha or Jehovah Tsidekhe.  Instead we pray as Jesus said to do so.  "Our Father....".  Jesus tells us that no good thing will He refuse us.  That includes healing if it is the Father's will.

The name comes from rapa'  "to heal." This word is common to both ancient and modern Hebrew. It occurs approximately 65 times in the Hebrew Old Testament, appearing first in Gen 20:17: "...God healed Abimelech."

"To heal" may be described as "restoring to normal," an act which God typically performs. Thus, appeals to God for healing are common: "...O Lord, heal me; for my bones are vexed" Ps 6:2; "Heal me, O Lord, and I shall be healed..." Jer 17:14. Not only are human diseases "healed," but bad water is restored to normal or "healed" 2 Kings 2:22; salt water is "healed" or made fresh Ezek 47:8; even pottery is "healed" or restored Jer 19:11.

A large number of the uses of rapa' express the "healing" of the nation-- such "healing" not only involves God's grace and forgiveness, but also the nation's repentance. Divine discipline leads to repentance and "healing": "Come, and let us return unto the Lord: for he hath torn, and he will heal us..." Hos 6:1. God promises: "For I will restore health unto thee, and I will heal thee of thy wounds, saith the Lord..." Jer 30:17. Even foreign cities and powers can know God's "healing" if they repent Jer 51:8-9.

(comments from Vine's Expository Dictionary of Biblical Words, Copyright (c)1985, Thomas Nelson Publishers)