NOLOCK or Read Uncommitted locking / latching behaviours
Reads under READ UNCOMMITTED isolation only take Sch-S. No shared locks are acquired (there are a couple of rare exceptions).

Reads under locking READ COMMITTED take S locks that are usually released as soon as the query processor moves on to the next row (again, there are some exceptions).

Pages are always latched while being read or written to ensure physical consistency, generally a shared latch on read, and an exclusive latch on write. For details see Inside SQL Server Latches by Bob Ward via PASStv.

Locks are an implementation detail used by SQL Server to deliver the guarantees provided by the isolation level.