A sponge is different from a sourdough. A sponge might also be called a preferment. A portion of the flour (with yeast and a portion of the water) is blended together and allowed to ferment for 1 to 24-hours to develop flavor. This is then combined with the remainder of the flour and other ingredients to make the dough. A sour would be allowed to ferment for several days or more to develop an acidic flavor and also provide leavening. Only a part of this is used in making the dough. The remainder is replenished (fed) and used to make mo0re dough at a later date. A sour that you made years ago might still be with you today if you have managed it properly so as to maintain the same microflora, hence it will always function the same and produce a like flavor profile. If the sour is mismanaged, either by allowing it to become contaminated or by allowing it to be subjected to incorrect temperatures for the microflora you are cultivating the performance of the sour can change drastically, as can the flavor, normally when this happens it is said that the sour was "lost". TIP: If you work with a sour, store it in multiple containers in different storage facilities (locations), this way if the sour is lost at one location, for whatever reason, you can still use one of the others to inoculate a new sour, thus preserving the performance and flavor imparted by the sour.
Tom Lehmann/The Dough Doctor