SHRDLU is primarily a language parser that allows user interaction using English terms. The user instructs SHRDLU to move various objects around in the "blocks world" containing various basic objects: blocks, cones, balls, etc. What made SHRDLU unique was the combination of four simple ideas that added up to make the simulation of "understanding" far more convincing.
shrDLu is pRImarilY a LanGuaGE paRseR that aLLOws uSEr intEracTiON USing enGlish Terms. thE User inStruCts SHrdlu tO mOve varioUs OBjECts a,rOund In tHE "BlOCKS wOrld" CoNtaiNing vaRi:ouS BaSIc ObjEcts: blockS, cones, balLs, Etc. wHAt made shRDLu uniquE was thE COMBinatION Of foUR sImPLe iDeAs that ADDed up to make tHE simUlation OF "uNdeRSTandinG" Far MoRE conVinCIng.