Some folks have asked whether Cassandra or Riak in last-write-wins mode are monotonically consistent, or whether they can guarantee read-your-writes, and so on. This is a fascinating question, and leads to all sorts of interesting properties about clocks and causality.
There are two families of clocks in distributed systems. The first are often termed wall clocks, which correspond roughly to the time obtained by looking at a clock on the wall. Most commonly, a process finds the wall-time clock via gettimeofday(), which is maintained by the operating system using a combination of hardware timers and NTP–a network time synchronization service. On POSIX-compatible systems, this clock returns integers which map to real moments in time via a certain standard, like UTC, POSIX time, or less commonly, TAI or GPS.
The second type are the logical clocks, so named because they measure time associated with the logical operations being performed in the system. Lamport clocks, for instance, are a monotonically increasing integer which are incremented on every operation by a node. Vector clocks are a generalization of Lamport clocks, where each node tracks the maximum Lamport clock from every other node.