POP
POP stands for Post Office Protocol. POP is used to download email from the ISP provider's email server. Most ISPs offer either POP or the newer protocol, IMAP to subscribers. POP3 does not require the SMTP to send email from the subscriber's email client.

Telnet
The Telnet protocol is a service for internet service providers that provides a means to connect into the ISP. It is sometimes used for updating Web sites, and frequently used as a method of connecting into a shell account. It is however extremely insecure with how it handles passwords. More modern protocols like SSH should be considered instead.

FTP
FTP stands for File Transfer Protocol. It is an ISP service that essentially just provides a way to connect to the ISP. It is frequently used as a means of updating Web sites. It is however extremely insecure with regards to how it handles passwords. More modern methods like WebDAV should be considered instead. When shopping for an ISP, ones that do not offer more secure alternatives to FTP should not even be considered.

IMAP
IMAP stands for Internet Message Access Protocol. This protocol works like POP except that it keeps your email on the ISP provider's mail server until you decide to move it. IMAP service can cost more than POP because your account may take up additional disk space as your email remains on the ISP's email server.

Shell Account
Some ISPs offer shell accounts. A shell account is a special type of ISP service: it is essentially text-only access to the Internet. You may wonder why anyone would want such a thing. After all, while e-mail is mostly textual anyway, the Web is a different animal. Consider though that a blind user doesn't care about pictures on a Web site, but still may need its underlying information. Many blind users rely on shell accounts working in conjunction with special screen readers. Many users also enjoy the virus protection shell accounts provide.

X-Face
X-Faces were devised to make both e-mail and Usenet news a little more human-friendly. They are ISP-neutral and will work with any ISP provider that provides an e-mail and/or a Usenet ISP service. They do require a savvy e-mail / Usenet application to create and display them properly, though. When an e-mail message or Usenet news item has an X-face attached, it will be displayed as a small monochromatic image (usually somewhere in the header) by applications that support them when viewing the owning message or item. Sometimes they feature miniature portraits of the author, but more often they're used just to give a little insight into the author's personality.

APOP
The old POP3 protocol is probably the most popular ISP service for receiving e-mail. Unfortunately it is by default very insecure with regards to how it handles passwords. APOP is an option that fixes POP3's password insecurity, improving overall internet service provider safety and ISP security. Note though that it does nothing for e-mail privacy -- only e-mail security. Something like S/MIME or PGP is required to improve privacy, too. Note also that if SSL is being used, APOP isn't needed

SSL
SSL stands for Secure Sockets Layer. It is a special type of ISP service used to encrypt communications between an ISP and a user. This is done not just for privacy, but also ISP safety. Unfortunately not all ISPs support SSL yet, so check with an ISP provider before making any commitments. Keep in mind that while e-mail sent and/or received via SSL will be encrypted for its trip to and/or from the ISP, it will not be encrypted for the remainder of its journey and will thus still be potentially open to snooping. To really protect e-mail one needs something like S/MIME or PGP.

Internet Service Provider
There are many different types of ISPs (Internet Service Providers). There are local ISPs and national ISPs. There are ISPs that focus on different types of connections, like modem dial-in, ISDN, ADSL, DSL, or broadband. There are business ISPs and residential ISPs. There are even ISPs that offer different access methods, like graphical (PPP/SLIP) or text-only (shell). Be sure to compare ISPs before choosing one.

WebDAV
Some ISPs provide WebDAV access to hosted Web pages. WebDAV (sometimes called DAV) is a type of ISP service that can be used to mount ISP resources as if they were installed locally. It provides all the benefits of FTP, but is significantly more secure. While a discount ISP will typically not offer WebDAV, a better ISP provider usually will. Remember that a free ISP offering only FTP access will end up costing you money if your account is cracked.

Picons, AOL Buddy Icons, and .Mac Icons
PIcons, AOL Buddy Icons, and .Mac Icons are similar to X-faces, but there are a couple important differences. First, they're stored in a master database (in the case of PIcons this database can be mirrored by each individual ISP provider as a regular ISP service). Second, they're in color. Third, (in the case of PIcons only) they can represent companies and organizations in addition to individuals. Savvy programs can display PIcons, AOL Buddy Icons, or .Mac Icons with e-mail messages or Usenet news items instead of (or in addition to) X-faces. PIcons are an open standard while AOL Buddy Icons and .Mac Icons are proprietary and only available to certain ISP users.

SSH
SSH stands for secure shell. It is an ISP service used for connecting into the ISP. It is often used for updating Web sites, and heavily used as a method of connecting into a shell account. When shopping for a shell account in this day and age, only ISP providers with SSH should be considered. A free ISP offering only Telnet access will end up costing you money if your account is cracked.

SMTP
SMTP stands for Simple Mail Transfer Protocol. SMTP is used to send email from one mail server to another. SMTP work in conjunction with POP to send and receive email. Some ISP providers do not use SMTP because of spam issues, which may overtax their servers.

Syndication
Better ISPs provide an interesting ISP service: the ability to syndicate Web content. Calendar event type information can be syndicated through a technology called iCalendar. Journal entries or news event type information can be syndicated through either a technology called RSS (RDF Site Syndication) or a related technology called Atom. While traditional syndication is text-only, due to the popularity of the iPod music player more and more podcasts (RSS with attached audio files) are appearing. Many dynamic Web servers have the built-in capability to automatically syndicate appropriate content making the whole process effortless.