I have worked out your code.I found few mistakes there ,

To check the exit or quit command from the user just you just check using the buf content using strcmp.
Also when I execute your code I got some warnings .You used gets() to get the input,but it is not a good practice always use fgets().Then you checked the single character with the NULL.to check the single character use '\0'.

Here is the code .

Code:

       #include 
#include 
#include
main ()
{
        char buf [1024];
        char *args [64];

        for (;;)
        {
                /*
                 *prompt for and read a command.
                 */
                printf ("trial_shell:");

                //   if (gets(buf) == NULL)
                if(fgets(buf,sizeof(buf),stdin)==NULL) // It is better than fgets
                {
                        printf ("\n");
                        exit (0);
                }

                /*
                 * Split the string into Arguments.
                 */
                buf[strlen(buf)-1]='\0'; // To remove the new line

                parse (buf, args);

                /*
                 * Execute the command.
                 */

                /* Check it here ,if it is exit or quit come out from the shell */
                if((strcmp(*args,"exit")==0)||(strcmp(*args,"quit")==0))
                {
                        exit(0);
                }

                execute (args);
        } // endof for loop
}

/*Simple help function that dumps help text to the screen*/

void help()

{
        printf ("*******trial_shell usage/help options********/n");
        printf ("help - prints the list of commands supported by trial_shell/n");
        printf ("cp   - copy a file (cp source target)\n");
        printf ("del  - deletes a file del (del file\n");
        printf ("host - prints the hostname of the machine\n");
        printf ("cd   - changes the directory (cd dir1)\n");
        printf ("clr  - clears the screen\n");
        printf ("dir  - lists the contents within a directory\n");
        printf ("help - displays the help associated with your shell\n");
        printf ("quit - exits the shell\n");
        printf ("environ - lists the environment settings\n");
        printf ("echo - displays the comment followed by a new line\n");

}
/* parse -- Splits the command in buf into individual arguments */

parse (buf, args)
        char *buf;
        char **args;
{
        while ( *buf != '\0') // Since *buf has single character compare it with '\0'
        {
                /*
                 *Strip White Spaces. Use Nulls, so
                 *that the previous argument is terminated automatically
                 */
                while ((*buf == ' ') || (*buf == '\t'))
                        *buf++ = '\0';

                /* Save the Argument. */

                *args++ = buf;

                /* Skip over the argument. */
                while ((* buf != '\0') && (*buf != ' ' ) && (*buf != '\t'))
                        buf++;
        }

        *args= NULL;
}

/* execute -- spawns a child process and execute the program */
execute (args)
        char **args;
{
        int pid,status;

        /* Get a child process */

        if ((pid= fork()) <0)
        {
                perror("fork");
                exit (1);
        }

        /*
         * The Child executes the code inside the if
         */
        if (pid == 0)
        {

                if (strcmp(*args, "help")== 0)
                {
                        help(); /*call the help function*/

                }

                else
                {
                        execvp (*args, args);
                }
                perror (*args);
                exit (1);

        }
        /* The parent executes the wait. */

        while (wait (&status) !=pid)
                /*empty */;

}

Thanks