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Concepts :-

The Spiritual Life: Prayer in Islam can be divided into 4 basic forms:  canonical prayer (solat) supplication (dua) litany (wirid) invocation (Dzikr)

Solat has 2 forms - mandatory (fardh/wajib) and sunat (recommended/optional).  The mandatory solat must be performed according to strictly defined  rules at specified times., while the sunnat solat can  be performed anytime except at some specific forbidden time such as after asr and before maghrib.

Dua:-

As for dua one may "supplicate" God at any time and in any circumstances without any pattern or formula. Supplications are strictly voluntary and "free". Some of the most intimate glimpses of the pious Muslim soul are to be found in supplications!

Wirid:-

Wirid involved recitation of Quranic formulae or one or more of the Names of God - like supplication these are voluntary, although they are not so "free" since they follow a set of patterns, like the canonical solat, which must be in Arabic. 

Dzikr:-

While Wirid may be performed by any pious muslim, dhizkr are recited almost exclusively by the Sufis.

Although supplications left by the great saints of early Islam are of the type of "free prayer", invariably they have one element in common:

since they are recited in Arabic (although they can be made in any language), they are largely inspired by Quranic images and incorporate Quranic verses and formulae. Also they are rhythmic and very often - employ rhymed prose (saj').