"echo" and "cat" are two commands.
Two completely different commands, which do two entirely different things.
echo - display a line of text
echo [OPTION]... [STRING]...
cat - concatenate files and print on the standard output
cat [OPTION] [FILE]...
Concatenate FILE(s), or standard input, to standard output.
cat: echo: No such file or directory
The first example just "echoed back" what I typed - the letters "c", "a" and "t"
The second example tried to print the contents of the file "echo". But there was no such file "echo" - hence the error.
'Hope that helps