in brief, lignans are oligomeric while lignin is polymeric..they are both phenylpropanoids based...detailed explanation may be found at: https://en.wikipedia.org/wiki/Lignan;

http://www.life.illinois.edu/ib/425/lecture9.html

http://www.sciencedirect.com/science/article/pii/S0168945214002003;

https://www.crcpress.com/Lignin-and-Lignans-Advances-in-Chemistry/Heitner-Dimmel-Schmidt/p/book/9781574444865

Thanks to all your answers were informative.

Lignin and lignan are closely related in term of their chemical structures, but they are different characteristics. Lignin is a polymeric substances as one of the cell wall components of woody plant. Lignin has no optical activity. 

While lignan is a group of phenylpropanes dimers, where the phenylpropane units were linkages by the central carbon (carbon number 8) of their side chains, as one of the extractives located mainly in the lumen cell. Lignan has optical activity.

Lignin is the most naturally abundant and important biopolymer substance in plant cell walls, exceeded only by cellulose. Its main function is to impart rigidity to the cell walls and, acting as an intercellular binder, renders wood the ability to withstand considerable mechanical stress such as resistance to compression, impact and bending. Lignin constitutes up to 40% of forest fibre and up to 30% of certain agricultural residues. Lignin is an amorphous, highly branched polyphenolic macromolecule of complex structure with high molecular weight. The chemical structure of lignin is highly irregular and extremely challenging, consisting of primarily of phenyl propanoid units (mainly p-coumaryl alcohol (H), coniferyl alcohol (G) and/or sinapyl alcohol (S)).7,8 These components are cross-linked in three dimensions through a radical coupling process occurring during its biosynthesis. Lignin’s physical and chemical properties are highly dependent on the wood species, botanic region and the isolation processes. They may be stored as glycosides lignins of conifers/softwoods are based mostly on coniferyl alcohol lignins of hardwoods are mostly based on coniferyl and sinapyl alcohols lignins of bamboo/grasses, palms mostly are based on p-coumaryl alcohol. Lignins are (organic compounds) complex non-carbohydrate aromatic polymer present in all wood. Lignins are class of complex organic polymers that form important structural materials in the support tissues of vascular plants and some algae. Lignins are particularly important in the formation of cell walls, especially in wood and bark, because they lend rigidity and do not rot easily. Chemically, lignins are cross-linked phenolic polymers.

The lignans are large group of chemical compounds found in plants. Some examples of lignans are pinoresinol, podophyllotoxin, and steganacin. Lignan are any of a classes of phenylpropanoid (propylbenzene) types of molecules found in essentially all plants, generally dimeric or higher order, and produced by secondary metabolic pathways branching off of aromatic amino acid biosynthesis, in some cases having associated antioxidant or estrogenic (phytoestrogenic) activities; having in common with lignin the phenylpropanoid monomers, where lignin is a random oxidative polymerization of the same.

Thanks to all to clear my question

In brief, Lignin: 1) lignins are crosslinked phenolic polymers

2) posses no optical activity

3) Provides structural support and conducting water transport in plants

4) Example of lignin: lignosulphonates and kraft lignins

while lignan

1) lignans are dimeric or oligomeric polyphenolic compounds

2) Posses optical activity

3) Non structural in nature but play role in plant defence against biotic and abiotic stresses

4) Example: Pinoresinol and podophyllotoxin.

You can checkout the following link for deeper understanding

Article Lignin and Lignan Biosynthesis: Distinctions and Reconciliations

Differences between primary and secondary metabolites

Lignans and lignin differ in their molecular weight, the former being small and soluble in water, the latter being high polymers that are undigestable. Both are polyphenolic substances derived by oxidative coupling of monolignols.