It's just like Shelly says.
Tile Layer and Hex Editor are the tools that's the most used tools and techniques for everyday ROM hacking.
Tile Layer tools lets you redraw the tiles and sprites on any ROM that can show decrypted 2D sprites. YY-CHR is a good Windows-based program that's super-easy to use. I use that for my VixyNyan ROM Hack, and I can do it with other ROMs too. ^^
If you want to go deeper, and hack the text, music, palette colors, and more, you need a Hex Editor, like Hex Workshop. There are special hex-numbered values that you change inside the file, and that gives special result.
You can even rip music and stuff if you copy a long string of code and then paste it into a new file. With the right codec, that file will become a playable sound file.
For most of the ROMs, like DS ROMs or PSP & Wii ISOs, you have to extract the files or folders that are inside them. For DS ROMs, you can do that with NDSheader. Then you use a Hex Editor to edit the things inside the file you opened. If you have enough luck, you can find easily-readable text that you can change, and reinsert into the ROM later.
When you want to translate the text in a ROM, you can do that with a table (.tbl) file. In those files, you can add any letter or symbol, and next to them, you write down the hexadecimal number the ROM is using for that letter/symbol. When you open the .tbl file in a Hex Editor that supports it, you will be able to read a game's text much easier, and can change it anyway you like.
I hope that helps a little bit. ^.^
I can give more help when I'm on my laptop. </wii>