Most popular services are WWW and eMail, based on protcols called HTTP, SMTP and POP3. Other well known services are FTP, Usenet (news - protocol NNTP), Telnet and IRC. If you don't understand what words like "service" or "protocol" mean, just picture all this stuff as alternative or enhancment to well known electronical communication services like televison, voice phone or fax.
There are also another experimental services, like Internet Phone, See-You-See-Me, PointCast network, ICQ or Internet Radio, which are not generaly understood as Internet standard. Some of Internet protocols are rather historical, like Gopher or Finger. History of Internet is very interesting chapter, if you are interested in it, try to get some book about Internet.
Most important among TCP/IP protocols are TCP (transmission control protocol),IP (internet protocol) and UDP (user datagram protocol). Pieces of data transported by TCP/IP are called packets.
Advanced operating systems (like Unix) implement TCP/IP as basic system service (on the same level as other I/O operations); in small systems like DOS, TCP/IP must be integrated to the application. This is why configuring Arachne is so confusing: you are configuring both TCP/IP and browser (and if you are using modem, than also a PPP dialer) in one program! You are provided with complete network solution, which together with operating system fits one floppy and can boot from it.
TCP/IP was designed for permanently connected computers; if you want to connect using phone line and modem, you will need to establish SLIP or PPP protocol between your Internet provider and your computer to transfer TCP/IP packets to your computer.
Arachne is WWW client - program which can request, download and view HTML page. WWW uses protocol called HTTP.
Only mass communication technology as easy to use as World Wide Web is probably television. Users of world Wide Web are expected only to be able to read and to know how to use mouse to select hypertext links, nothing more.
Electronic mail is simply exchange of text messages (which can contain attached encoded files) between Internet users. Arachne can currently send and recieve simle eMail messages, using protocols called SMTP (sending eMails) and POP3 (downloading eMail from mailbox). Every user of internet mail is assigned unique address, in format username@domain. Your username does not have to be identical to your login name. The domain is ussualy (but not always) a name of your provider's mail server.
Having personal eMail address is today probably as common, as having personal phone number.
The URL syntax is protocol://[username[:password]@]hostname[:port][/URI]
URI is shortcut for Uniform Resource Identifier, and it is in fact very often just a filename of document or program situated on WWW server.
OpenDOS is latest member of Digital Researh's CP/M development line: CP/M ... CP/M ... DR-DOS ... Novell DOS ... Caldera (DR)OpenDOS.
But you are likely to have 386 or better processor, which can be switched to "protected mode" (also called "32bit" mode) and access all memory. From DOS, you can do this for example by using DPMI driver. You are probably asking, why Arachne is real mode application, running only in the first 640 kB of your PC's memory: it's because golden era of DOS was on real mode computers and most development tools, libraries and helper applications feel best in real or "V8086" (also called "Virtual") mode of processor. Developing for protected mode DOS is much more complicated task.
Arachne, as real mode application, is very small program, which fits on single floppy: compare this to protected mode applications, written in object-oriented languages, which are ten times bigger and you are downloading them from Internet for ages. Arachne was developed mainly for PCs with 80286, 80386 or 80486 compatible CPU, with only several MB of RAM, where real mode DOS applications perform much better than Windows applications. On Pentium P5/P6 (trademark of Intel), AMD K5/K6 or various 80586 and 80686 compatible machines with more than 8 MB of memory, real mode applications are not able to use the full potential of the system. However, you can use extended memory manager like EMM386 or QEMM to optimize "virtual DOS machine" on your 386 or better computer to be very efficient and actually much faster then operating system which are expected to run on these machines. Using DOS on Pentium class computers can give you probably more "real-time" feeling then using any other OS.
Check your system manual or configuration utility for information how to configure RAM disk on your PC.