There are quite a few types of Raspberry Pi. I tried to summarize simply from the old type to the new type.
The differences roughly are in functions and versions.
|PRIMER VERSION||GPIO EXTENDED||CPU ENHANCED||Wi-Fi and Bluetooth INCLUDED|
|Series A||Pi MODEL A||Pi MODEL A+||N/A||N/A|
|Series B||Pi MODEL B||Pi MODEL B+||Pi2 MODEL B||Pi3 MODEL B|
|ZERO||N/A||Pi ZERO||N/A||Pi ZERO W|
The primer version has been discontinued, so the current models are six types. According to WikiPedia, there is the Compute Module. This model is excepted from the list because it is very special.
The MODEL B as a high grade is evolving more than the MODEL A as a cheap edition.
The ZERO has no LAN port similarly the MODEL A, in addition no camera module and the smaller USB and HDMI ports. However, its CPU clock is higher and its memory size is same as the MODEL B. It is a pointed model of the MODEL A.
Which model do you choice?
If you want to embed the Raspberry Pi, you should choice the ZERO equivalent to the MODEL A++. Its size is a half of MODEL B.
Now, the most popular model is Pi2 or Pi3.