Here are some important terms used generally in telephony and specifically in FreeSWITCH. What's also needed is a broad but precise conceptual overview, written for the layman. The terms can sometimes be confusing and trip up understanding; hopefully this glossary will help.
enCoder / Decoder - software that translates data in/out of some format. Particularly: audio data-length compressors / decompressors.
Set of XML files that parse the call details and decide where to route the call.
Telephony protocols such as SIP/H.323 and POTS lines.
From pg. 74 of the FreeSWITCH 1.0.6 book:
The counterpart to having a user register to your FreeSWITCH server is to have your server register as a user on a remote server. This is accomplished using gateways. A gateway is quite simply an outbound registration to another SIP server. Telephone service providers use very large servers to provide SIP trunks to their subscribers. In FreeSWITCH, we can use a gateway to connect to a SIP provider. We can also use a gateway to connect to another SIP server, such as another FreeSWITCH server or any SIP-compliant IP-PBX.
From pg. 79 of the FreeSWITCH 1.0.6 book:
In the strictest sense of the word, a SIP profile in FreeSWITCH is a User Agent. In practical In practical terms, this means that each SIP profile "listens" on a particular IP address and port number. The internal profile listens on port 5060, and the external profile listens on port 5080. Not only does the profile listen but it can respond as well. For example, when a phone sends a SIP REGISTER packet to FreeSWITCH (at port 5060), the internal profile "hears" the registration request and acts accordingly. The files in conf/sip_profiles/ are ones which determine how the profiles behave. Many of the parameters in these profiles are to customize how FreeSWITCH handles various SIP traffic scenarios. In most cases the defaults are reasonable and should work. In other cases, though, you may find that because of the peculiarities in various VoIP phones and vendors, you will need to make adjustments. Lastly, do not let the profile names internal and external be a source of confusion. Each profile is simply a user agent that is streamlined for a specific purpose. The internal profile is optimized to handle telephone registrations and calls between registered phones. The external profile is optimized for outbound gateway connections and several NAT traversal scenarios.
For a deeper discussion of user agents and the concept of a back-to-back user agent (B2BUA) see wikipedia article about this.