Within raids the Holy Paladin is mostly used as a tank healer, as he is somewhat lacking in group healing capabilities in contrast to the Druid or Priest. With his Flash of Light and Holy Light the Paladin is best suited to heal single targets, especially if they take lots of damage. Let's take a look at the spells at the Paladin's disposal before diving further into the healing strategy.
Flash of Light has a casttime of 1.5 seconds, is extremely mana efficient and gets 43% bonus from spellpower. A Holy Paladin can cast this spell literally forever, as the mana regeneration is usually high enough to counter the small mana cost of this spell.
The second healing spell in the Holy Paladin's arsenal is Holy Light. With a 2.5 seconds cast time, which can be talented down to 2.0 seconds, it is only slightly slower, but heals for a lot more. This huge throughput comes at a hefty increase in mana cost though. This makes the spell especially useful to counter damage spikes. One of the core talents for a Holy Paladin is Illumination. Illumination refunds 60% of a spell's mana cost if you score a critical strike. There are several ways of playing a Holy Paladin. The most famous way is the Flash of Light spammer, exclusively using this spell and relying on the fast cast speed and constant stream of healing. The low mana cost basically nullifies any mana concerns, enabling the Paladin to concentrate on stats like critical strike rating, haste and spellpower. Haste has a huge effect on healing throughput, which makes it a prime concern for this type of healing. This provides a constant stream of healing for the Tank and is efficient snipe-healing for the raid.
A Holy Light Paladin will cast his big heal more often, utilizing the Glyph of Holy Light for some splash AoE-healing while keeping the tank alive with Beacon of Light. This type of Paladin will still cast the occasional Flash of Light, as Holy Light is way too expensive, but his big Holy Lights are the spells that count.