MyISAM Vs. InnoDB

1. InnoDB recovers from a crash or other unexpected shutdown by replaying its logs. MyISAM must fully scan and repair or rebuild any indexes or possibly tables which had been updated but not fully flushed to disk. Since the InnoDB approach is approximately fixed time while the MyISAM time grows with the size of the data files, InnoDB offers greater availability and reliability as database sizes grow.
2. MyISAM relies on the operating system for caching reads and writes to the data rows while InnoDB does this within the engine itself, combining the row caches with the index caches. Dirty (changed) database pages are not immediately sent to the operating system to be written by InnoDB, which can make it substantially faster than MyISAM in some situations.^{[citation needed]}
3. InnoDB will store rows in primary key if present, else first unique key order. This can be significantly faster if the key is chosen to be good for common operations.^{[citation needed]} If there is no primary key or unique key InnoDB will use an internally generated unique integer key and will physically store records in roughly insert order, as MyISAM does. Alternatively, an autoincrementing primary key field can be used to achieve the same effect.
4. InnoDB currently does not provide the compression and terse row formats provided by MyISAM, so both the disk and cache RAM required may be larger. A lower overhead format is available for MySQL 5.0, reducing overhead by about 20% and use of page compression is planned for a future version.
5. When operating in fully ACID-compliant modes, InnoDB must do a flush to disk at least once per transaction, though it will combine flushes for inserts from multiple connections. For typical hard drives or arrays, this will impose a limit of about 200 update transactions per second. For applications which require higher transaction rates, disk controllers with write caching and battery backup will be required in order to maintain transactional integrity. InnoDB also offers several modes which reduce this effect, naturally leading to a loss of transactional integrity guarantees, though still retaining greater reliability than MyISAM. MyISAM has none of this overhead, but only because it does not support transactions.
6. MyISAM uses table-level locking on writes to any existing row, whereas InnoDB uses row-level locking. For large database applications where many rows are often updated, row-level locking is crucial because a single table-level lock significantly reduces concurrency in the database.
7. MyISAM is still widely used in web applications as it has traditionally been perceived as faster than InnoDB in situations where most DB access is reads. Features like the adaptive hash index and insert buffer often mean that InnoDB is faster even if concurrency isn't an issue.^{[citation needed]}
8. Unlike InnoDB, MyISAM has built-in full-text search.

read more at : http://en.wikipedia.org/wiki/Myisam