How different between these 2 connection Strings, one has pooling = false, the other is not. ?

Question

1. <add name="MyConnect" connectionString="server=.;database=OnlinePostOffice;Pooling=false;Connect Timeout=45;uid=sa;pwd=456"/>
2. <add name="MyConnect" connectionString="server=.;database=OnlinePostOffice;uid=sa;pwd=456"/>
If I use the second String, sometime I got this error:
"Timeout expired. The timeout period elapsed prior to obtaining a connection from the pool. This may have occurred because all pooled connections were in use and max pool size was reached."
Now I must use the first String. But I don't know how different between us.