There have been some U.S. Special Forces in Yemen for some time. Indeed a group was attacked in Aden. However training of Yemeni forces was suspended during the time when there were negotiations for Saleh to step down. Now the U.S. supported president Hadi is in power military aid has resumed and now troops are being sent as well.
The troops will cooperate with Yemeni security forces in counter terrorism operations. No doubt we will see renewed attempts to retake territory captured by militants in southern Yemen. Any militant Islamis group now seems to be described as Al Qaeda or linked to that group. AQAP is the official Al Qaeda group in Yemen.
The new U.S. troops will work under secret arrangements. At the same time drone attacks are being increased. Targets have also been broadened with signature strikes based upon behavior now being allowed. Targets now need not be specifically identified. This broadening will no doubt increase the number of civilians casualties. For more see this article.