You hear both terms used to refer to broadly the same action.
Seizing is the taking. Impounding is a possible action after the seizure. Goods may be impounded, or forfeited, or destroyed, or turned over to the legal owner, or something else.
If you're hearing both terms referring to broadly the same action, it's because the cases you're hearing about are broadly the same: something is seized and impounded.
Seizing describes the act of taking an item from its (supposed) owner or controller. It has no length of time ascribed to it. For example, the following things are seizing:
- The policeman takes the keys of a motorcyclist to prevent them from fleeing during a traffic stop.
- The police investigating the murder take a knife used to stab a victim to do tests on it.
- The police take the firearm of the felon away to be destroyed.
Impounding describes the act of storing a seized item until it is released or sold off for non-retrieval.
- The phone seized from the student using it in class is impounded on the teacher's desk/the secretary office/...
- The car seized from the drunken driver is stored at the impound lot.
Every act of impounding requires a seizure, but not all seizures are impounding the item.