The word Shloka or Padya (=poem) has a wide meaning. It primarily means a poem in any of the 26 meters in Samskritham or in any combination of them.
In a confined sense, it means a simple poem (or stanza) in the 8 th meter, named Anushtup.
Hence a shloka is often referred to as Anushtup too.
Its basic structure is as follows. Remember here, a letter is a syllable in itself. A short syllable is called Laghu (=small,short)and a long one, Guru (=long, big) ,each represented by a separate symbol of its own, | and U respectively.
It has four lines. A line is called paada,meaning one fourth. The first 2 lines form the first-half and the remaining 2 lines form the second-half. As far as possible, the sense shall be complete in each half.
In all, 256 different compositions are possible for a shloka.
However, deviations from these rules are found in epics and works of great poets. At any rate, the rythem of the rendition should be pleasing to the ear. That is the rule paramount.
In the Mahaabhaarataham, however, many poems of other meters were also used alongwith some prose text , but all are commomnly called shloka's
Each line contains 8 letters .
In every line, the 5th letter (=syllable) should be a short one (laghu).
In the odd lines, i.e. in the 1st and 3rd lines, the 6th, 7th and the 8 th syllables should be long ones ( guru).
In the even lines i.e. in the 2 nd and 4 th lines, the 7 th syllable should be a short one, while the 6 th and the 8 th syllables should be long ones.
The 2 nd and the 3 rd syllables in any line should not be simultaneously short. There is a convention in prosody to treat the line-ending laghu as guru.