The paper presents initial results from a James Webb Space Telescope (JWST) survey of Cassiopeia A (Cas A), the youngest Galactic core-collapse supernova remnant. The survey includes near-infrared (NIRCam) and mid-infrared (MIRI) imaging mosaics that map emission from the main shell, interior, and surrounding circumstellar/interstellar material (CSM/ISM). Additionally, four exploratory positions of MIRI/MRS integral field unit (IFU) spectroscopy are used to sample ejecta, CSM, and associated dust from representative shocked and unshocked regions. Key findings include: 1. **Web-like Network of Unshocked Ejecta Filaments**: A web-like network of unshocked ejecta filaments is observed, resolved to scales of $\sim 0.01$ pc, consistent with turbulent mixing of cool, low-entropy matter from the progenitor's oxygen layer with hot, high-entropy matter heated by neutrino interactions and radioactivity. 2. **Dust-Dominated Emission from Shocked CSM**: A thick sheet of dust-dominated emission is seen in the remnant's interior, pockmarked with small ($\sim 1'$) round holes formed by high-velocity ejecta knots that have pierced through the CSM and driven expanding tangential shocks. 3. **Light Echoes**: Dozens of light echoes with angular sizes between $\sim 0'$1' to 1' are discovered, reflecting previously unseen fine-scale structure in the ISM. NIRCam observations place new upper limits on infrared emission ($\lesssim 20$ nJy at 3 $\mu$m) from the neutron star in Cas A's center, providing constraints on scenarios involving a possible fallback disk. These JWST survey data and initial findings address unresolved questions about massive star explosions, with implications for the formation and evolution of stellar populations, metal and dust enrichment of galaxies, and the origin of compact remnant objects.The paper presents initial results from a James Webb Space Telescope (JWST) survey of Cassiopeia A (Cas A), the youngest Galactic core-collapse supernova remnant. The survey includes near-infrared (NIRCam) and mid-infrared (MIRI) imaging mosaics that map emission from the main shell, interior, and surrounding circumstellar/interstellar material (CSM/ISM). Additionally, four exploratory positions of MIRI/MRS integral field unit (IFU) spectroscopy are used to sample ejecta, CSM, and associated dust from representative shocked and unshocked regions. Key findings include: 1. **Web-like Network of Unshocked Ejecta Filaments**: A web-like network of unshocked ejecta filaments is observed, resolved to scales of $\sim 0.01$ pc, consistent with turbulent mixing of cool, low-entropy matter from the progenitor's oxygen layer with hot, high-entropy matter heated by neutrino interactions and radioactivity. 2. **Dust-Dominated Emission from Shocked CSM**: A thick sheet of dust-dominated emission is seen in the remnant's interior, pockmarked with small ($\sim 1'$) round holes formed by high-velocity ejecta knots that have pierced through the CSM and driven expanding tangential shocks. 3. **Light Echoes**: Dozens of light echoes with angular sizes between $\sim 0'$1' to 1' are discovered, reflecting previously unseen fine-scale structure in the ISM. NIRCam observations place new upper limits on infrared emission ($\lesssim 20$ nJy at 3 $\mu$m) from the neutron star in Cas A's center, providing constraints on scenarios involving a possible fallback disk. These JWST survey data and initial findings address unresolved questions about massive star explosions, with implications for the formation and evolution of stellar populations, metal and dust enrichment of galaxies, and the origin of compact remnant objects.