Join[$l_1$, $l_2$] Join concatenates lists:
Join[{a, b}, {c, d, e}]
Join[{{a, b}, {c, d}}, {{1, 2}, {3, 4}}]
The concatenated expressions may have any head:
Join[a + b, c + d, e + f]
However, it must be the same for all expressions:
Join[a + b, c * d]