A regular or normal SSL certificate provides encryption for one domain (and one sub-domain as ZNetLive SSL certificate covers both the WWW and non-WWW versions of your website).
In case, you are using a number of domains and subdomains for your business, you can go for Wildcard SSL. Wildcard certificate is issued with a common name of *.companydomain.com and a Subject Alternative Name (SAN) of companydomain.com. For example, if a SSL wildcard certificate is issued for *.znetlive.com, then it can be used for securing mail.znetlive.com, www.znetlive.com or anything.znetlive.com and also znetlive.com.
It saves cost as you buy only a single SSL certificate for multiple subdomains and with a single certificate, administration also becomes easier.