| Lindlar catalysts are poisoned palladium catalysts used in the hydrogenation of alkynes. They are useful because of two features of the accompanying reaction: it only hydrogenates once, giving an alkene rather than an alkane; and the alkene product is selectively the cis- or Z-isomer.
Alkynes contain a carbon-carbon triple bond, C≡C, so hydrogenation initially gives an alkene, with a C=C double bond. However these can be further hydrogenated to alkanes, containing the C-C single bond; using a Lindlar catalyst prevents this second hydrogenation.
Alkenes, meanwhile, are subject to geometrical isomerism: the hydrogens bonded to each carbon in C=C can either be on the same or opposite faces of the molecule, giving the cis- and trans-isomers respectively (or Z- and E-isomers). Lindlar catalysts ensure stereoselective hydrogenation to the cis-isomer.
The catalyst itself, like many inorganic catalysts, is a transition element, in this case palladium on a "support" of calcium carbonate, CaCO3. On addition of hydrogen, such a catalyst by itself would hydrogenate an alkyne all the way to the alkane, so the Lindlar catalyst is also "poisoned": this is usually done with lead, in the form of lead acetate, Pb(OAc)2, while other available poisons are barium sulfate, BaSO4, and quinoline, C9H7N. This is not biological poisoning, of course, but rather the "poison" acts to inhibit the catalyst so that after the first hydrogenation, the reaction proceeds slowly and can be easily stopped before hydrogenation is repeated.
In synthetic chemistry it is often desirable to hydrogenate alkynes to alkenes, and the question of geometrical isomerism in alkenes is key (as it always is). Therefore Lindlar catalysts are extremely useful.
For example, an important compound in perfumery is Z-jasmone, a rather complicated alkene ketone containing two C=C double bonds and a carbonyl group, C=O. It can be synthesised from the corresponding alkyne using a Lindlar catalyst, which ensures that all C=C and C=O bonds are untouched and that the product is indeed the Z-isomer: the E or trans-isomer has no use in perfumery. | 
