There are three different implementations of the Lightning Network, Eclair by ACINQ, LND by Lightning Labs, and C-Lightning by Blockstream. From a high level, you can think of Eclair as a business friendly implementation, LND as a community driven and developer friendly implementation, and C-Lightning as a reference implementation.
The fundamental technical differences of the implementations are based around the coding language. Eclair is based on Scala, LND is based on Go-lang and C-Lightning is based on C.
An important thing to note is that all the implementations are compatible with one another. While there are three implementations, there is just one Lightning protocol.