S7 is the current series of Siemens PLCs (as opposed to older S5 PLCs). The S7 Series is further broken down into groups based on the size and functionality of the PLCs.
The low end product is S7-1200, which programmed with a software package called Step 7 Basic. The S7-1200 is an eventual successor to the wildly popular S7-200 micro PLC line.
The S7-200 requires a program called MicroWin, but because the S7-200 was not originally developed by Siemens (it was a Texas Instuments product), the software is totally different.
The S7-300 is the nid-range product and is the most popular for most machine and small process control applications.
The S7-400 is the high performance / high I/O count product line. It's hardware is also used in their PCS-7 DCS systems as well.
The program that is used for Siemens S7-300 and S7-400 PLCs, is called Step 7, often mistakenly called "S7". It comes in 3 or 4 different packages based on the complexity and functionality you need for your project, the top of the line being the Step 7 Professional.
There is a new software bundle called "TIA Portal" that puts the Step 7 package together with the HMI programming software and integrates other product programming software under an overall umbrella to allow Totally Integrated Automation (TIA), the new "buzzword" for Siemens. You can get the Step 7 Basic and all other levels of Step 7 integrated into TIA portal, but not the older MicroWin product, they are totally different.
WinPLC7 is NOT a Siemens product, it is made by another German company called Vipa. Vipa makes a knock-off copy of Siemens PLCs. Their software is purported to work on real Siemens PLCs, but if you use it, you get zero support from Siemens if you have a problem. Siemens' support structure is based on the software packages they sell, not the hardware. So if you do not have a registered copy of the Siemens software, you get no support just because you bought their hardware. I have never used a Vipa PLC or their software so I have no opinion on their product, other than from the perspective of an ex-Siemens employee.
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I am new to Siemens myself... have fun! And may the forced bits be with you... it is the most annoying PLC software, and PLC language I have ever had to deal with!
That summary was good though... one thing that was left out is you can get Step 7 Lite... it is kind of like the Microsoft Express Line of .Net development... I don't know how watered down it is (I think one of the hooks is... it will only work with a limited model range of models of PLCs... AND if you don't have a PLC to work with to work though exercises in a learning curve there is a PLC emulator available from Siemens called either SIMPLC or PLCSIM... but it's not free.
I have yet to find another PLC emulator/simulator for practicing particularly one that is free. IF SOMEONE KNOWS A GOOD ONE... please let me know.