The Raspberry Pi is a microcomputer board with an ARM core in a System On a Chip (SoC) with RAM piggybacked on top of it in 256MB to 1GB configurations, and having some form of network connectivity (with the exception of the RPi Zero and RPi 1 A+). It is expected on those units that you will add a network dongle if you need to get to a network.
There are several versions of the Raspberry Pi board that are available at the time of this writing. I believe the following is a complete list. Several versions may fall off in January 2020, due to obsolescence.
|Model||# Bits||# Cores||CPU speed||RAM||ETH||Wireless||Bluetooth||Form Factor|
|Raspberry Pi 3 Model B+||64||4||1.4GHz||1024MB||1GB||2.4GHz/5GHz||4.2/BLE||Standard|
|Raspberry Pi 3 Model A+||64||4||1.4GHz||512MB||-||2.4GHz/5GHz||4.2/BLE||Compact|
|Raspberry Pi 3 Model B||64||4||1.2GHz||1024MB||100Mb||2.4GHz||BLE||Standard|
|Raspberry Pi 2 Model B||32||4||0.9GHz||1024MB||100Mb||-||-||Standard|
|Raspberry Pi 1 Model B+||32||1||0.7GHz||256-512MB||100Mb||-||-||Standard|
|Raspberry Pi 1 Model A+||32||1||0.7GHz||256MB||-||-||-||Compact|
|Raspberry Pi Zero W||32||1||1GHz||512MB||-||2.4GHz||4.1/BLE||Zero|
|Raspberry Pi Zero||32||1||1GHz||512MB||-||-||-||Zero|
The 3A+, 3B, 3B+, Zero and Zero W are the current release versions, although RPF says they plan on continued production of the others until early 2020. I would still stick to the later versions, simply for their enhanced performance. It is not unreasonable to spend $45.00 on a computer and power supply to run a resource intensive application, so the 3B or 3B+ are great for computer-like problem solving. If you are solving a problem that doesn't require a full desktop, the Zero and Zero W might be a better match.