What stops me using more than 25GB on home and 50GB on data?

Quotas.

What the heck is a Quota?
A quota is a restriction that is placed on the amount of data an account may use.
A quota consists of a soft/hard quota and a grace period.
A soft quota is a predetermined level where a warning email is sent out. This however does not prevent you writing more data until the grace period is reached.
A grace period is the amount of time you are allowed to go over the soft limit before the system blocks your ability to write more data.
A hard quota is a predetermined level where the system blocks your ability to write more data. You will still however be able to read your data.

Could I have an example of the Quota system?:
Scenario 1 -
I have 20GB of data in my home directory. I copy another 10GB to my home directory.
So I now have 30GB of data which means I'm 5GB over the soft limit. I receive an email stating I have gone over the quota.
A counter is now started on the server which is called the grace period.
I now have 7 days to reduce my data back down below 25GB (the soft limit). However I can still keep my data at 30GB safely for a week.

Scenario 2 -
I have 30GB of data in my home directory and I decide to copy another 25GB. As soon as I reach 50GB of usage on my home directory, the system stops me writing anymore data.
The copy / scp / rsync command would return an error "quota has been exceeded".

Scenario 3 -
I have 30GB of data in my home directory and I forget to reduce the data below 25G. The 7 day grace period then expires.
The 25GB soft limit now becomes a hard limit. I will be unable to write anymore data to my home directory until I reduce my usage below 25GB.