What is the difference between InnoDB and MyISAM?

I am using MySQL as my database for my current web project. I am new to MySQL. Please explain to me the difference between InnoDB and MyISAM.

ISAM = Indexed Sequential Access Method and is essentially a flat file (for those DBAs who can remember, think Btrieve, or B-Tree). It's a very old technology - but don't let that put you off using it. Because it's a flat file (more on that later), it is not relational, and thus is not an RDBMS, and thus is more appropriate in some situations.

InnoDB is the full RDBMS like you are most likely familiar with. MyISAM can appear to be relational through another layer added on top that maintains your links, logic and referential integrity.

ISAM is brilliant if you have a LOT of records (like, 20 million), and the records are mostly stand-alone (i.e. you don't need to do lots of links to retrieve associated data). It relies VERY heavilly on indexes and if you don't have the right index, be prepared for very very long query times. Case in point: We had a Btrieve ISAM table with 20M+ records and to do a retrieve and filter data based on an accurate index was almost instant. Using the wrong index was literally 15 minutes.

InnoDB is great for if you have a lot of relational links. Table A references a field in Table B, which references Table C and D. InnoDB can fetch these records using all sorts of nice joining methods (hash joins, etc), whereas an ISAM database would have to run multiple sub-queries for every single row and match the records manually.

You will really have to do a course in databases if you want much more detail than that!

The most fundamental is that InnoDB is transactional. MyIsam isn't. MyIsam is generally a bit faster, so if you don't need transactions that is generally your best bet. For detailed descriptions you should read the MySQL documentation.