Discovering and segmenting objects in videos is a challenging task due to large variations of objects in appearances, deformed shapes and cluttered backgrounds. In this paper, we propose to segment objects and understand their visual semantics from a collection of videos that link to each other, which we refer to as semantic co-segmentation. Without any prior knowledge on videos, we first extract semantic objects and utilize a tracking-based approach to generate multiple object-like tracklets across the video. Each tracklet maintains temporally connected segments and is associated with a predicted category. To exploit rich information from other videos, we collect tracklets that are assigned to the same category from all videos, and co-select tracklets that belong to true objects by solving a submodular function. This function accounts for object properties such as appearances, shapes and motions, and hence facilitates the co-segmentation process. Experiments on three video object segmentation datasets show that the proposed algorithm performs favorably against the other state-of-the-art methods.