The TStringList(Or TStrings) is much like a fancy dynamic array or Set of Strings(Not Possible). It will come in handy alot when programming and I'm going to teach you basic TStringList Usage!
<Delphi> Program StrList;
Str := TStringList.Create;//This is needed when using this class(or most classes) Str.Add('Some String!'); Readln;
This is a simple program(Console) that will create and add one string to a string list! Now heres some things you should know:
Create - Will create the string list for modifying.
Count - Is a counter for the amount of strings in the List. It is a zero based counter
Add - Will allow you to add one string to the string list. It is a Function that will return the Index of the String. This is Were the counter comes in handy.
Delete - Will delete a string from the string list. Just know that you do not simply input the string, you have to input the index of the string.(Like I said. Its like a fancy Dynamic Array)
IndexOf - Will allow you to return the index of the string in the list. If it is not found it returns -1.
Clear - Will clear the list.
How about a more juicy example, Eh?
Str:TStringList; S:String; Counter:Integer;
Str := TStringList.Create; Writeln('String List Test'); Repeat Writeln('Input a string to add'); Readln(S); IF(S = 'EXIT')Then Halt; IF NOT(S = )Then begin Counter := Str.Add(S); Writeln('String: '+S'+ was Added!'); Writeln('Index is: '+InttoStr(Counter);//The counter will always become the index of the last thing added end Else Writeln('Nothing Inputed :D'); Until(S = 'EXIT');
When using the TStringList you have 2 main file handling procedures. SaveToFile And LoadFromFile By Using SavetoFile it will save all strings in the list to a file. LoadFromFile will open the file and add the file data to the list string by string.
Str := TStringList.Create; Str.LoadFromFile('SomeFile.txt'); Str.Add('Hello'); Str.SaveToFile('SomeFile.txt');
You just opened a file,edited it and saved it right back to were it was!
You can learn all the different procedures,functions and property's Here: 
IF you feel i might have left something out, Modify At Will!
Hope I helped!