Wildcard Certificate: a wildcard certificate secures unlimited subdomains. For example, if you order a RapidSSL Wildcard or True BusinessID Wildcard for *.yourcompany.com, the certificate will secure www.yourcompany.com, mail.yourcompany.com, hosting.yourcompany.com, login.yourcompany.com and other subdomains related with yourcompany.com.
UC/SAN: a UC/SAN certificate secures up to 24 domains that all belong to the same organization. The UC/SAN certificate cannot be ordered directly and it associates with True BusinessID and True BusinessID with EV. For example, you can order a True BusinessID/ True BusinessID with EV, and then add up to 24 domains as additional domains in Add Domains Field in your OnlineNIC control panel.