Why do ps -a and ps-a print out different results? What is the difference between adding a bar and not adding a bar?

  linux, question
monini@monini-ThinkPad-T410:~$ ps -a
 
 PID TTY          TIME CMD
 16837 pts/1    00:00:00 sudo
 16856 pts/1    00:00:00 su
 16857 pts/1    00:00:00 bash
 16891 pts/1    00:00:00 eclipse
 16892 pts/1    00:03:13 java
 16937 pts/1    00:00:00 dbus-launch
 17733 pts/1    00:00:10 java
 31743 pts/5    00:00:00 ps

The following is the result of ps a typing:

monini@monini-ThinkPad-T410:~$ ps a
 PID TTY      STAT   TIME COMMAND
 803 tty7     Ssl+  31:39 /usr/bin/X -core :0 -seat seat0 -auth /var/run/lightdm/root/:0 -nolisten tcp vt7 -novtswitch
 811 tty1     Ss+    0:00 /sbin/agetty --noclear tty1 linux
 16707 pts/1    Ss     0:00 bash
 16837 pts/1    S      0:00 sudo su
 16856 pts/1    S      0:00 su
 16857 pts/1    S      0:00 bash
 16891 pts/1    S+     0:00 ./eclipse
 16892 pts/1    Sl+    3:13 /usr/bin/java -Dosgi.requiredJavaVersion=1.6 -XX:MaxPermSize=256m -Xms40m -Xmx512m -jar /home/xujianxing/eclipse/eclipse//  plugins/org.eclipse.equinox.launcher_1.3.0.v20140415-20
 16937 pts/1    S+     0:00 dbus-launch --autolaunch=89513375dda449418c27b0cd9ece4e44 --binary-syntax --close-stderr
 17733 pts/1    Sl+    0:10 /usr/lib/jvm/java-7-openjdk-amd64/bin/java -Dfile.encoding=UTF-8 -classpath /home/monini/workspace/shiyu_Web/CommonRestService/target/classes:/home/monini/workspace/shiy
 21439 pts/5    Ss     0:00 bash
 31771 pts/5    R+     0:00 ps a

One is Unix style. One is BSD style. The meaning it represents is also different.