First off, you said you intended to change the default policy, and the command you provided (-A)ppends the rule, leaving the default ACCEPT in place. If you want to change default policy use the following:
iptables -D INPUT DROP
Secondly, there are 3 types of action iptables can take when parsing a packet:
ACCEPT - allow packet to continue as intended.
REJECT - block the packet from moving forward, send an ICMP message back stating that the connection was rejected.
DROP - diacard the packet completely with no notification
Rejecting rule is useful when you want to know that the packet reached the iptables layer and was discarded (which can be helpful when debugging the firewall). It is arguably more secure to use DROP, as in the real world scenario, as the attacker would not get any information on the state of the system ports.