Spline joints are a variation of the tongue and groove joint. Instead of a fixed tongue it has a loose spline and is installed by driving the spline in from the end or cutting off the bottom of the groove and dropping in place. This is sometimes necessary in wood floor repairs.
I've also used this joint when lining up casework panels. When there isn't enough room to install the final panel with a fixed tongue and groove then a spline joint can be used. Making a loose spline is easy with a table saw and cutting the grooves can be done with a router and 1/4" router bit.