WLCG is made up of four layers, or "tiers"; 0, 1, 2 and 3. Each tier provides a specific set of services.
This is the CERN Data Centre, which is located in Geneva, Switzerland. All data from the LHC passes through the central CERN hub, but CERN only provides around 20% of the total compute capacity.
Tier 0 is responsible for the safe-keeping of the raw data (first copy), first pass reconstruction, distribution of raw data and reconstruction output to the Tier 1s, and reprocessing of data during LHC down-times.
These are thirteen large computer centres with sufficient storage capacity and with round-the-clock support for the Grid. They are responsible for the safe-keeping of a proportional share of raw and reconstructed data, large-scale reprocessing and safe-keeping of corresponding output, distribution of data to Tier 2s and safe-keeping of a share of simulated data produced at these Tier 2s.
The Tier 2s are typically universities and other scientific institutes, which can store sufficient data and provide adequate computing power for specific analysis tasks. They handle analysis requirements and proportional share of simulated event production and reconstruction.
There are currently around 160 Tier 2 sites covering most of the globe.
Individual scientists will access these facilities through local (also sometimes referred to as Tier 3) computing resources, which can consist of local clusters in a University Department or even just an individual PC. There is no formal engagement between WLCG and Tier 3 resources.