Unlike the regular Tarrasch, in the Semi-Tarrasch Defense Black does not accept an isolated pawn, since he intends to recapture on d5 with the knight (as after 5.cxd5, 5...exd5 has long been known to be dubious after 6.Bg5), but he cedes a spatial advantage to White.

After 4...c5, White usually plays 5.cxd5 Nxd5 6.e3 or 6.e4, which leads to different types of middlegame play and has attracted the interest of strong players with both colors since the early twentieth century.

Wikipedia has related information at Semi-Tarrasch Defense.