• It refers to a user device, and can be either a smartphone or a USB dongle used for data communication purpose only (i.e., a USB type which is connected to a laptop for wireless Internet).  
• Each UE has a “USIM card with built-in IMSI required for identification/authentication of subscribers” that can be inserted in it.
• It also has a built-in LTE chip and thus can attach to an LTE network.

• An eNB is called an "LTE base station" and is an equipment that provides radio connection between a UE and an LTE network.
• In the Figure above, only the connection between the UE and the eNB is wireless and all the rests are wired (through IP network).  

• For S-GW, it is hard to find a word that can describe all of its various features at once. However, one of the most representative characteristics will be the fact that it serves as an anchoring point for inter-eNB handover. When a UE attempts to disconnect from eNB1 that it has connected to for Internet use and connect to eNB2 (this is called a handover, and communication is not interrupted during this handover), an S-GW serves as an anchoring point for the handover from eNB1 to eNB2. As you know, you drop an anchor to make something stay in one place. So, what it means is, the S-GW is used as an axis during the handover of a UE from eNB1 to eNB2. 
• Or you can say it in style, "S-GW serves as an anchoring point in intra-LTE mobility".

• A P-GW is very versatile.
• First, it assigns an IP address to a UE (through the initial attach procedure for the UE defined in 3GPP, not through DHCP protocol).
• Second, we just learned that an S-GW serves as an anchoring point for eNBs. Similarly, a P-GW works as one as well, but for S-GWs. When a UE switches from S-GW1 to S-GW2 as it travels (from an eNB managed by S-GW1 to an eNB managed by S-GW2), a P-GW serves as an anchoring point.
• Third, it applies different QoS policies per UE (i.e., performing priority, bandwidth control, etc.).
• Fourth, it manages accounting data per UE. Accounting data generally include information such as uplink/downlink traffic volume (# of bytes, # of packets), access times, etc. A P-GW delivers these data, in Charging Data Record (CDR) format, to an OFCS. In other words, it generates and manages all the information including the time at which each subscriber attaches, the amount of data the subscriber uses, and the time period during which the subscriber stays connected, and forwards the same to an OFCS.

One thing to note, user data travel through a UE, eNB, S-GW and P-GW to communicate with the Internet. So, no user data travels through other network entities (e.g. MME, PCRF, etc.).

MME (Mobility Management Entity)

• An MME serves as "brain" of the LTE network, performing following duties:
• First, it authenticates a UE using EPS-AKA protocol. As key information needed for authentication of the UE is stored in an HSS, the MME performs authentication of the UE, on behalf of the HSS, using the key information received from the HSS.
• Second, it  manages EPS bearers, logical paths (logical channel and GTP tunnels) established along the route of {UE - eNB - S-GW - P-GW} in order to let a UE to connect to the PDN (e.g., the Internet). It is in charge of establishing, modifying, terminating, etc. of the GTP tunnels.
• Third, it manages the mobility of a subscriber. That is, it checks whether the subscriber's UE is currently attached to the network or not, and if it is, whether the UE is using the Internet (in connected state) or not (in idle state).    

HSS (Home Subscriber Server)

• An HSS is a central server DB of an LTE network that contains (1) key information needed for authentication, and (2) subscriber profiles of each UE. 
• In subscriber profiles, QoS class  information (e.g. priority, maximum bandwidth available, etc.) specific to the service subscribed by each user is stored.
• When a UE attaches an LTE network, an HSS delivers key information needed for authentication and user profile to an MME. Then, the MME, the brain for the LTE access network, authenticates the UE and generates an EPS bearer based on the user profile (subscribed QoS information).

PCRF (Policy and Charging Rule Function)

• A PCRF defines rules for policy and charging to be applied to each UE. QoS information to be applied to a UE is determined according to policies, and information on whether to charge offline or online is determined based on charging rules. That is, the primary function of a PCRF is defining rules for QoS and charging for each UE. The PCRF forwards these rules to a P-GW, which then performs controlling (QoS, charging, etc.) of the UE based on the rules forwarded.

SPR (Subscriber Profile Repository)

• Policy and charging rules (access profile) of each UE is not stored in a PCRF, but in a DB called SPR.
• So, the PCRF retrieves a UE's access profile from the SPR as needed.

OCS (Online Charging System)

• Most of mobile communication service providers offer postpaid services in Korea. However, quite a lot of them provide prepaid services in other countries. 
• For example, if a user who purchased a "1GB prepaid card valid for one month", the network should be able to manage the data usage of the user in "real-time" and stop the Internet service when the 1GB is used up (or the period of one month is expired). And that is what an OCS is for.
• A P-GW monitors the real-time usage and forwards the same to an OCS. The OCS, then centrally manages the usage available (called balance or credit) per user to identify those who have no more balance or credit, and informs the P-GW so that it can stop the Internet service for them.

OFCS (Offline Charging System)

• As explained earlier, an OFCS is a system that centrally manages CDRs forwarded by a P-GW.

PDN (Packet Data Network)

• You can think of it as the Internet. For now, let's just say PDN = Internet = IP Network.