Lindlar catalyst is a "poisoned" metal catalyst that performs hydrogenations of alkynes in the presence of hydrogen gas (H2). By "poisoned" we mean that it lacks the normal activity associated with palladium catalysts for reducing double bonds. This is useful because sometimes we'd like to start with an alkyne and go down one "rung" of the oxidation ladder to an alkene. But if you use normal palladium on carbon, you'll get full reduction to the alkane.
Lindlar's catalyst is a palladium catalyst poisoned with traces of lead and quinoline, that reduce its activity such that it can only reduce alkynes, not alkenes. It always gives the cis-alkene, in contrast to Na/NH3, which gives the trans alkenes. Lindlar's catalyst doesn't really have a "structure". Like Raney nickel, it's basically a metal that has been modified in a very particular way to provide a certain desirable set of properties. Sometimes you might see it written as Pd-CaCO3-PbO2, but it's usually just written "Lindlar".
Similar or the same as: There's a whole family of poisoned catalysts that are similar. You might also see Ni–B (nickel boride), Pd-CaCO3, palladium on barium sulfate, Pd-CaCO3-quinoline and others enlisted to do the same task.
Examples: Reduction of alkynes to cis-alkenes
How it works
It is thought that the role of lead (Pb) is to reduce the amount of H2 absorbed, while quinoline helps avoid the formation of unwanted byproducts, but it's hard to be more specific than that.