This information is mostly taken from Stephen Hawking's lecture, 'The Direction of Time', and some other scattered online sources. It's a bit too long to write out in full, but I'll do my best to simplify it.
"The past is a foreign country. They do things differently there - but why is the past so different from the future? Why do we remember the past, but not the future?" - L.P. Hartley
Physics describes the universe as being T Symmetric, i.e. all laws are time symmetric. Despite this, we see an obvious difference between the past and the future. Hawking describes the different ways in which the direction of time flows, which he calls 'time arrows'.. There are four different time arrows, each of which dictates the flow of time in its own way:
- The thermodynamic arrow of time is based on the Second Law of Thermodynamics - put simply, the overall disorder (entropy) of the universe is always increasing with time. From this, we can say that time is pointing in the direction in which disorder is increasing.
- The causal arrow of time describes that time is pointing in the direction in which cause precedes effect. This is an effect of the thermodynamic arrow.
- The psychological arrow of time describes the past as that which we remember, and the future is that which we don't. This arrow describes time as pointing in the direction opposite to that which we remember. This can be reduced to the thermodynamic arrow because the energy used to embed memory increases the amount of disorder in the universe.
-The cosmological arrow of time is that time is pointing in the direction in which the universe is expanding and not contracting.
There are more arrows (radiative, quantum etc.) but I'm trying to keep this short and they are all consequences of the thermodynamic arrow. It can be shown that the thermodynamic, causal, and psychological arrows always point in the same direction. Hawking discusses if these will all reverse if the cosmological arrow reverses (i.e. when the universe starts contracting again), but research by Raymond Laflamme has shown that if the universe contracts, it need not be time symmetric.
If this is the case, all four arrows will always be pointing in the same directioin ('forwards' as we percieve it') in relation to the universe.
I'm not sure if this makes sense to you guys, I've tried to summarise it but it's hard to write and explain as skilfully as Hawking. Let me know if you need anything clarified.