Delphi is a product of Borland International. It is a native code compiler that runs under Windows 3.1 and provides visual programming tools somewhat similar to those found in Microsoft Visual Basic 3.0. The underlying language is Object Pascal, which is an extension of the object-oriented Pascal found in Turbo/Borland Pascal starting with version 5.5.
Delphi has been available in beta-test for many months now, and Borland has also given away a large number of "prerelease" copies. As far as the official release is concerned, though, there are two packages: Delphi (sometimes referred to as Delphi Desktop) and Delphi Client/Server. Both are version 1.0.
The only version of Delphi that has been released is for Windows 3.1. There is no reason why it should not run correctly under systems that provide Windows 3.1 emulation, like OS/2 Warp, Windows NT, etc. Borland has announced plans for a 32-bit version to coincide with Windows 95. It is rumored that this might be a free upgrade to users of Delphi 1.0, but I wouldn't count on it. It is also known that Delphi 1.0 does not run correctly on the prerelease version of Windows 95.
Applications built in Delphi are Windows 3.1, 16-bit applications. However, Borland has stated that existing Delphi applications will compile unmodified in 32-bit Delphi.
The minimum installation of Delphi takes about 30Mb, and the full install takes 80Mb. In order to run it well, you'll need a 486 with a minimum of 8Mb of RAM, though I personally wouldn't try to run it in less than about 12Mb. I use a 486DX2/66 at home and a Pentium-90 at work, and to be honest, there's not much difference between them--Delphi's compiles are so fast that the CPU is really not a bottleneck.
If you're lucky, you already have lots of experience with both Object Pascal (or, as it used to be called, Borland Pascal With Objects--essentially, Turbo Pascal v5.5 or later) and with Visual Basic. If you fit this description, then Delphi will be a breeze for you.
Okay, now for everyone else. In order to make full use of the Delphi environment, you have to know Pascal, you have to have some grasp of object orientation, and you have to understand event-driven programming. Once you're over those three hurdles, you've pretty much got it. See section 5 for more information.
On the other hand, most people don't need to make "full" use of the environment. If you just want to pull a simple application together that doesn't do anything too fancy, Delphi shouldn't be any harder to learn than VB--it's just that there's a whole lot more you *can* do in Delphi, which will make you feel more lost than you really are.