The mechanism for locking shapefiles has changed with ArcMap 10.0. Lock files are created in the file system and correspond to the particular shapefile and to the type of lock required by the system. These .lock files are automatically created and deleted at runtime. Normally, they do not remain in the file system once the application has been closed (e.g., ArcMap, ArcCatalog). But these files can occasionally remain following a crash. If this is the case, they can be safely deleted. If a .lock file cannot be deleted, it is a valid, live lock, and some process is still actively using the shapefile.
Note: This new locking mechanism for shapefiles now conforms to one that has been in place for file geodatabases since version 9.2.Below are some examples of schema locks on shapefiles.