This is one of the more complicated tech sections largely due to the fact that forestry - growing, cutting, and selling tree products - is a complicated business with specialized kit. We break the kit up into its logical sections to make this easier for you to figure out.
Your cutter basically replaces the Chainsaw for the purposes of cutting down the tree as it does so in an automated manner - however - the Cutter does NOT replace the Chainsaw. That is to say you need to keep your Chainsaw and not sell it since you still need to use it for the process of trimming branches. Just saying.
This type of kit makes use of pallet-loaded saplings - it will allow you to load it with sapling pallets and then easily plant them.
This device type attaches to the tractor and is operated as an accessory.
This unit is used solely for transporting logs. Some other units perform several functions like loading and transporting.
The following pretty awesome new additions to this section were added to the base game as paid DLC...
Notes
Bear in mind that you can get into Forestry without needing a million dollars... All that is technically required to start in is a Chainsaw and a loader/hauler combined with one of your existing Tractors.