Yes, there are and it is vital two understand the difference in order to be able to navigate a boat successfully.

True North:

True North is an imaginary straight line between you and the geographic North Pole. This straight line is a great circle that passes through you and both the North and South Poles. It is called a Meridian of Longitude.

Magnetic North:

The Magnetic North Pole, that spot the needle or card on your compass points towards, is not located at the geographic North Pole.
Magnetic North as detected from Portugal is located south west of the geographic North Pole. As a result, there is an angular difference between True North and the direction your compass is pointing. This angle is called Variation and in Portugal magnetic north is a little more than 3º west of true north, this angle will vary a little according to your position.

To find out how much variation there is in an area that you intend to sail in, look at a local chart. There will be two red circles, one inside the other (see the picture above) they are called a Compass Rose. Each circle is marked in one degree increments from 000º to 359º. The 000º on the outer circle is pointing towards True North, the 000º on the inner circle is pointing towards Magnetic North.

Magnetic North is moving!

The variation for the area is printed in the middle of the compass rose, it will be shown in degrees and minutes (there are 60 minutes to a degree) either east or west of true north. There will also be a date shown and the figure in minutes to the right of the date is the amount that the variation is moving every year and E or W to show whether it's moving to the east or west. You can then make a simple calculation so that you know the up to date variation for that area.

The Compass Rose shown above I scanned from a chart of the western Algarve that I bought in 2007. The variation is shown as 3º 50' West in 2005 (variation shown on a chart is very rarely up to date for the year that you have bought the chart), to the right of that information is printed (9'E), which means that magnetic north is moving 9 minutes east every year. It is now 2008 (3 years later) and so magnetic north has moved 3 X 9' which comes to 27 minutes which deducted from 3º 50' leaves us with a variation of 3º 23' West for 2008. Accuracy with minutes is not important ans so I always round up or down to the nearest amount of degrees.

I am therefore now using a variation of 3º west for the Algarve and so to convert a course of 090º True to a Magnetic course it is clear from looking at the magnetic rose that a magnetic course will be greater and so 93º Magnetic.

I went to New Orleans to sail a few years ago and the compass rose on a local chart showed that there was no variation there at that time, which made life very simple for navigators there. However I was also in the Caribbean recently and the variation was shown as being 14º West and so when steering 090º True we had to steer 104º Magnetic.

If you were sailing on the west coast of America, the variation there is easterly (Magnetic North is to the east of True North). If you calculated that the variation is 5º east, then a magnetic course will be 5º less than a true course. So a course of 090º True will be 85º Magnetic.
Newcomers to navigation often find all this very confusing, but it doesn't have to be. Most navigation Instructors teach their pupils a mnemonic to assist them in knowing whether to add or subtract variation when converting from True to Magnetic or Magnetic to True. This I find helps about 70% of pupils, but the remaining 30% remain confused! When I was learning navigation I fell into the second category! I don't teach it using a mnemonic because it is my opinion that doing so creates an entire generation of navigators who know how to allow for variation without actually understanding what they are doing!

My way is as follows: Imagine a compass rose, we all know that the degrees run in a clockwise direction from 0º to 359º. If magnetic north is west it is to the left of true north, and as the magnetic compass also works clockwise, it increases clockwise and as it has started before true north any magnetic bearing or course must be greater than a true bearing or course and any true bearing or course will be less than a magnetic bearing or course.

There will be some people who have read this article and thought 'this doesn't apply to me, I navigate using GPS'! Well it does because your GPS can be set up to show either True or Magnetic bearings and courses, you need to know which it is showing and be able to change it from one to the other.